I'm not so sure they were forced to. I thought that too, initially, but another commenter somewhere else pointed out how hasty and nervy Google's "maps" announcement was, just before WWDC. In that context, it doesn't sound like something Google was wanting to happen.
What we do know is that Google raised their general prices for map access since the original deal with Apple was made. If we assume Google also wanted to increase the rate for Apple's access, it seems logical that they would be upset that they lost a multimillion dollar deal.
