First thing you need to know is that rep is 100% meaningless. I have seen people that I know are good guys, and they have negative rep, and I have seen total assholes with 9001 rep.
It doesn't mean a thing.
If you get dereped for a stupid reason, then does it really matter? No, it doesn't. Odds are rep will be gone soon enough anyway. And besides, you have an actual reputation, not just some silly boxes.
So yes, people can rep and derep you without a reason, or with a stupid reason, but it doesn't matter. If you really care about it that much then get people to like you, back people up in arguments, etc.
War: DJ is cool now days, havn't you heard? D: