Yeah, it's a mystery. Artis quote from above in this thread:"Mike's not a nice human being as far as I'm concerned and he knows that. ...
Mike and I will never be eye-to-eye, and he knows that. It's not gameplay. It's not gameplay, it's personal."
While Artis made it pretty clear in his interviews how he feels about Mike, and that he confronted Mike about it, he never did say what issue came up. Perhaps this was because he wanted to take the high road since Mike can't tell his side of the story at this point.
Pete takes that same road in not revealing the nature of the issue, but it is notable that he thinks Mike never understood.