Even amongst yordles, Rumble was always the runt of the litter. As such, he was used to being bullied. In order to survive, he had to be scrappier and more resourceful than his peers. He developed a quick temper and a reputation for getting even, no matter who crossed him. This made him something of a loner, but he didn't mind. He liked to tinker, preferring the company of gadgets, and he could usually be found rummaging through the junkyard.
Rumble showed great potential as a mechanic, and his teachers recommended him for enrollment at the Yordle Academy of Science and Progress in Piltover. He may very well have become one of Heimerdinger's esteemed proteges, but Rumble refused to go. He believed that Heimerdinger and his associates were sellouts, trading superior yordle technology to humans for nothing more than a pat on the head while yordles remained the butt of their jokes.
Source: The Publisher Website.