100% a power gap you have to reach first. VEGETA was an elite Saiyan, it took longer for him to become SSJ. Goku got that angry before, but this time he was a lot stronger, and it boosted him into the power needed to become super Saiyan.

So yeah, after a while when things are getting stronger, you'll likely get SSJ after an RP that leaves you enraged, or you RP a training as VEGETA did.