My opinion: Randall was always second best to sulley in MI so in MU when sulley was kicked out of the scare program randall was the best. He wanted to prove he was good because he was always a nerd and pushed around. when the ROR accepted him,he knew he couldnt be friends with mike cause they'd kick him out and it was the first time he ever felt accepted so he became a jerk just like them. He was pressured to change just to fit in.
Then when sulley 'screwed up' his scare he swore revenge cause he thought it was on purpose. Thus the asshole randy we all know today.