It was probably something he said that broke the rules.
Dean is actually a pretty 'no bullshit' kind of dude when it comes to warnings/punishments, it was actually one of the very first things I noticed when I became a moderator. It looks like Dean warned him twice today and he probably realized the dude wasn't going to listen so he just banned him, if I had to take a guess.
Dean has a very very forgiving ban policy, though. It normally requires like a single email with an apology for him to reconsider and unban you. Not always, mind you, but most of the time it works.