Downvotes usually signify “you posted something rude, objectively incorrect, off-topic, or instance/community rule-breaking” and people who think they did not do anything wrong might understandably be frustrated when they are getting the signal they did something like that, but they can’t figure out what bad behavior they’re getting downvoted for at all. They might want to know what the downvoter thinks they are doing that is inappropriate so they can stop it, or if it is just a troll trying to make people unhappy and thus ignorable.