Much had passed in the last orbital cycle. After breaking his way through the compound (his captors would likely have a lengthy repair bill for their crippled blast doors), Grimlock had finally thought he would escape. Instead, he was forced to face off with a copy of himself. The battle was legendary, with both combatants giving it their all. In fact, it was so amazingly epic that the writer couldn't fathom the words to describe it. While the fight was good, in the end Grimlock was able to defeat his clone with the aid of his new form. Simply put, he was a lot tougher than he was before.
After finishing his fight, he thought he was invincible. Wrong! His captors quickly showed that they were cowardly, installing some kind of weapon inside his frame to damage him should he resist. In the interest of self preservation, he chose to go along with them. Of course, he would make his discontent very evident, and made it perfectly clear what he would plan to do to them once he found a way out of his predicament.
Things happened, and he found himself on Cybertron once more. Apparently they were sending him out on some kind of test run. This angered him immensely, as if he was some kind of pet they could just sick on their enemies. Of course, his anger turned into reluctant willingness when he found out who his target was. He couldn't deny hunting down the bug Starscream would be amusing.