I feel like this is the sort of situation that shouldn't even be featured in the discussion. Who else signed the press report? It's totally irrelevant.
My guess is that Forstall refused to sign off on the public apology. Nobody was asking him to sign the letter, but he was asked by Cook to stand behind it. Failure to do so was probably the last straw for his remaining in an active role at Apple (a role he clearly was at least contemplating leaving after his big stock sale).
That's their justification. I'm still skeptical. No offense, but an apology not signed by Cook isn't an Apple apology. Tim was gonna' sign it no matter what I believe.
It would have been signed by Tim Cook on behalf of everyone at Apple, but my guess is that Forstall wasn't on board, regardless of whether his name appeared on the public announcement or not.