I think it was because Cochran's fear of the purple rock isn't the only reason he flipped -- although it certainly was one of them and the reason that EPMB chose to show. I think two other reasons were just as important.

The first is that Cochran could get along better with most of Upolu than he could with most of Savaii. Most of Savaii are young and cool or at least could relate to the "cool kids" mentality as Jim probably can. Cochran is a nerd. Period. He isn't part of the "cool kid" crowd and never will be. And he probably can't relate to the "cool kid" crowd. He can relate more to the strategic people -- who are mostly on Upolu. And maybe like Jonathan, he wanted the strategic people to win more than the "cool kids" -- even Dawn.

And although the "cool kid" crowd might have been pleasant to Cochran and tried to be nice, the fact is that they probably can't relate very well to him either. They're more focused on challenges -- especially Ozzy. And as I said, Cochran was focused more on strategy. The "cool kids" might have tried to be nice to him but people aren't as great as actors as they might think they are and we all know how good an actor Ozzy is. So they couldn't really conceal the fact that they didn't like Cochran all that much, especially Ozzy. And Cochran knew it. Thus bitterness probably played a role as well.