I just noticed that I lost 10 points of rep because "user was removed". I don't get it.
When a user is removed, their votes are removed as well. So the most likely explanation is that the user had voted for one of your answers and the reputation you gained from that vote was removed with the user.
The cause for deletion is usually either sock puppeting and vote fraud, or the user reqested to be deleted. In cases of high-reputation users with lots of votes, SE performs a different procedure that retains the votes.
This is one of the consequences of the recent upgrade to stackexchange software. Maybe this aspect of the updated software doesn't always seem like an upgrade. This happens from time to time on all stackexchange sites. I once lost more than 200 reputation points on m.s.e. because of just one user being removed. Who the user is, and which of your questions and answers are affected, is strictly secret, and that secrecy has been defended at length by a number of people.