I assume they ban by region, and if it just so happens you're online while they're doing your region, you get smacked.
I only play @ night since I have school & work during the day, and a friend of mine texted me saying he had just been banned while playing MW2, and surely enough, when I got home that night and turned on the box, my profile signed in, but I was not signed in to live.
Week earlier when MW2 first P2P leaked, same thing happened to other friends.
IIRC same happened with November/Saints Row 2/COD5 ban wave last year.
AND with the Assassins Creed/COD4 ban wave the year before that.