Hmm, I guess I should be more precise, and refer to the converstation between Lady Olenna and Ser Jaime. I wonder if she is referring to the garrison that defended Highgarden.

Lady Olenna: "Did we fight well?"
Ser Jaime: "As well as could be expected."
Lady Olenna: "It was never our forte, golden roses indeed."

So, lets go one point at a time. MC = Monochromatic Rainbow. KK = Kingkonrad.

1. Being caught off guard - MC, I'd definitely agree there, but it is a bit more indepth that, "Holy smokes Golden Rose defenders, a massive army." Lord Tarly defecting certainly didn't help, along with who knows how many other lords with him, as he is one scary old war dog. And I mean scary. I want to say Jaime said he was on par with Tywin, as a military leader. On that same note, of being surprised, the Lannister army would have had to march south completely undetected, again, a bit fishy.

2. The Tyrell's have bite to them, due to their numbers/ The Tyrell armies aren't a paper cannon- they are 80,000 men - MC and KK, I agree that numbers do help, but the episode's dialogue, and perhaps a bit of bias from myself, I would argue that the Tyrell's go for a "quantity is its own quality" mentality. Like the Soviets of WW2. You know, the enemy will run out of tanks and bullets before we run out of men. Don't take this as I am saying they are all untrained rabble and peasants, but rather, with having so many more men, I would argue that the need for more stringent/rigorous training is not as pressing. In the books, the Tyrell forces guarding the Shield Islands get overwhelmed very quickly. I digress, the point I am trying to get at is the Tyrells, while having numbers on their side, may not be the strongest army. 80,000 is a whole ton of men, but numbers aren't everything.

3. Highgarden probably had a minimal garrison at best, with the Tyrell forces were off somewhere completely different, mustering to repel a more... conventional assault - rather than something that deep within their territory. - MC, with Dorne to the South, and an ally, the Tyrells/the Reach wouldn't have to station a large portion of their forces there. Due to bordering both the Westerlands and the Crownlands, it would make sense to have the bulk of your forces to be stationed there to 1) Guard against attacks, and 2) Prepare for an offensive operation. I still wouldn't leave Highgarden with a minimal garrison if all me gold was there. I mean, that is a metric ton of bloody gold. Like, more gold than Yukon Cornelius could ever fathom. That would be like leaving a five man team to guard Fort Knox.

To me, the most reasonable explanation is that the Lannisters did a blitzkrieg strike. They abandon Casterly Rock, blitz south as fast as possible, brush aside any defenses in place, and just slam the hammer home on Highgarden, with its probably standard garrison. Lord Tarly defecting doesn't help as well, along with Tywin's training. Also, who knows how many other Reachmen either defected, or simply just said "F it, I'm going home." After the incident at the Great Sept, losing member of the High Command never helps. And Mace being the putz he was, yeah, it also didn't help.

4. KK mainly. Different timelines from our RP to the Show, to the Books - 100% Agree. I was saying that the main branch of House Tyrell went bleh, with Mace, Margery, Loras, and Olenna dead. Garlan and Willas are still kicking about in the books and this timeline, and I would say in the show too, but they are just either never mentioned, or get a brief, "Yeah, I had other kids too, but they didn't make my beautiful grandchildren Loras and Margery." The books also depict the Tyrell family being very large, so I am not worried that they died out, but rather, "Damn, the Queen of Thorns and her favorites are no more." Bummer. Them thieving Lannisters stole all your gold, sacked Highgarden, and stole Lord Tarly. Its would be cool if that was present in the current timeline for the RP, as it would make the love interest between Alerie and Tom that much more family blood feud angry status.

5. The Tyrell's were lordless - KK and MC, Mace dead, and who knows if any other lords with him at the Great Sept. Lord Tarly defected, with who knows what other Lords. Willas and Garlan are alive, no doubt, but they may have been abroad in the field, and not at home with their respective armies. I'd argue that the Tyrell forces suffered a bit of interior disarray from defections, combat losses, Wildfire losses, and god knows what else, which may have left the garrison defenses in either a less seasoned officer, or a grizzled old timer. A scrapping the barrel scenario. Also, the Tyrell armies may have lost men as well from any offensive or defensive operations.

6. Last, I promise - KK and MC, still a great episode, and it was awesome to see Lady Olenna tell Ser Jaime the truth, and go out like a boss. Anyhow, no worries, all speculation anyhow, but it is fun to try and figure out what the hell happened.

On the whole Tyrell situation, I feel that was a hell of a send-off for the Queen of Thorns, she will be missed. I believe the Lannister victory over the Tyrells can be attributed to

1) Randyll Tarly is probably the finest strategist living in the Seven Kingdoms, him working together with this new pragmatic and cunning Jaime would absolutely beat whoever the Tyrells had in charge of the garrison, probably a standard Master-at-arms.

2) The Lannister army is the most hardened in the Seven Kingdoms after all of the constant wars and the tight ship that Tywin and Kevan ran. The Tarlys would have made up the core of the Tyrell's considerable army, but with Tarly's defection and the defection of other Reach Lords presumably, you're talking about an army upwards of ten thousand men who would be much more hardened and battle ready than the Tyrell garrison. The Tyrells probably already had most of their forces deployed elsewhere in preparation for the coming battles and weren't expecting an attack so close to home. Even Jaime says they fought as good as the circumstances could allow.

Other thoughts

3) This doesn't mean the Tyrell family is gone, just Olenna and her favorites. If we were folding this into our own canon, we could say that Willas was either sent away or taken prisoner by Jaime only to be freed and Garlan was in the field with the main Tyrell forces, and couldn't do anything to save Olenna. The Tyrell House is also very expansive and the family is spread all over the Reach, the ones in Oldtown or different holds would have escaped the battle and even if they didn't they probably would have just been taken prisoner. Personally, I feel we should wait until the season ends to get a full picture of the political situation and think how that can be integrated into our own canon.

4) That said, I do like the idea of folding the Siege of Highgarden and death of Olenna into the backstory. It would add a lot more weight to the troublesome implications of Tommen and Alerie's forthcoming marriage, and would believably fuel a lot of political tension in both the Reach and the West and among Houses Lannister and Tyrell. Dickon Tarly, was just following his father's orders and likely would have bent the knee to Willas and the Crown after the defeat of Cersei's coalition and his father's assumed death in the war. This could mean tension between House Tarly and House Tyrell of course too.

5) This defeat as a whole is supposed to even the playing field between Daenerys and Cersei, taking it from where it was looking like a curbstomp in Dany's favor to Cersei actually dominating so far.

Admittedly, I always imagined the Tyrell forces as being something of a quantity over quality as well - but with a solid backbone of knights and trained retainers. The Tyrells are rich, if say... 5-10k of those 80-100k Reachmen are truly well trained, high quality soldiers, they can do the heavy lifting. You use your rabble to absorb some losses, pad the line - and if they falter, you have your shock troops ready to prop them up.

Also, the Soviets didn't really go quantity over quality to that extent. It's a common notion that Russia merely drowned their enemies in bodies - they usually did have the numerical advantage, but never to the extent of being like the hordes of vodka loving lemmings many people like to portray them as.