This is either, he's used an exploit/glitch in a game to benefit from it/make other people lose or, this is the weird phenomenon of people losing games and deciding they need to message their opponent afterwards.
One of your big games is usually involved in this(CoD, FIFA, Online fighting game of choice). I'd bet on the latter. Now I'm not shit hot on FIFA, but even I would receive many messages from people after I had won telling me how shit I am, or how they actually deserved to win. To get a lifetime ban he must have done this a lot.