there doesn't have to be a motive. DDoS doesn't only exist for retribution.
if you want to play competitive video games at a high level, this is just something that you'll need to keep in the back of your mind at all times.
instructions for protecting yourself for TF2 found here:
http://teamfortress.tv/forum/thread/5840-ddos-attacks
please keep in mind that these guys have lists of everyone's ip, so before you mask you should change your ip. masking won't do anything if they already have your ip documented.