Millions of years ago, before Optimus Prime and his Autobots left on their voyage that would leave them stranded on Earth, the rebellious Grimlock and his gang snuck into a Decepticon spaceport and stole a whole fleet of ships. Why? Why not! They were bored. In case Grimlock decided to secede from the Autobots and form his own group, he stowed them in the Rad Zone, where nobody'd look.
There they'd stay, until after the chaos-bringer Unicron had ravaged their planet, Cybertron, and the Decepticons abandoned the Autobots on their dying world. Plucking the stolen ships out of storage, Grimlock led the Autobots after the Decepticons to Klo, where the Decepticons were defeated and scattered.  Not satisfied, Grimlock retrofitted one surviving ship and dubbed it the Graviton, assembled a crew consisting of his Dinobots and a few others, and hunted Decepticons down, one by one.