I have had this happen a couple of times, or at least that's what I can gather. Someone, usually a relatively new user that has been on just long enough to downvote, answers the same question as me. My answers in these cases had nothing disagreeable about them and were otherwise pretty well-received. But my best guess is that in order to make their answer look better and rise to the top, they downvote my answer. Downvoting answers costs 1 reputation, but shouldn't it cost more if you have an answer on the same question? Maybe 4?
The martial arts StackExchange site has no control over this policy. There is a lot of network-wide consideration that has gone into the current settings over the years, and it is unlikely these will be changed.
See, for example: Should the weight of downvotes be increased?