@Nameless that happens, some people are just having some issues and go on a down-voting rampage. If it occurs excessively there are systems in place to detect it and revert it. Repeat offenders can also be suspended. It has happened to me as well on occasion, down-voting sprees for no obvious reason or feedback, although that user has now been removed. Interestingly that single user stood for all but one of the down votes I had o_O
Anyway, it might feel a bit annoying, but you gain nothing from caring about it. Just continue to contribute, and people will appreciate what you do.