I"m going off of two theories here. First is that your school uses a authentication system that requires you to either enter a password or register a MAC address to get access. The other, is that you were torrenting and the school got notified about it. From my own experiences in my college's IT department I can say that we got at least one letter a weak about a student torrenting telling us to stop them or face a lawsuit. Along the same line, if your dorm computer got a virus and was spreading it through the network they could have cut you off too.