I'll wait for some indication beyond 'Wenger didn't like Song's attitude in training' before I believe Song's attitude was why this sale was allowed. That would be a pretty low threshold of tolerance—not many other clubs would trash a useful player on that basis without exhausting other options.
My main concern is that it's happening because Wenger wanted to make further changes (eg Sahin) and was told he needed to flog a player to do it.
The other possibility that stands out for me is that this might be some kind of anti-Dein/anti-agent measure, following through on a policy about tapping up. I would like that to be true.