I think there are a few grumpy types that downvote for little reason, or perhaps completely arbitrarily. They're often the first votes in, as this action requires little consideration, and presumably such people have few other distractions.

Also, some downvotes are cast in error. This is very easy on touch-screens, and occasionally a problem elsewhere. Since totals are no longer displayed, a person may not even realize their intended upvote registered as a downvote.

So assume it was a mistake. Or a hopelessly grumpy person whose bad attitude is best ignored, rather than amplified with curses and complaints. Brush off the first downvote or two, and well-considered upvotes follow soon enough.