Why do some people downvote something without any explanation?

It is something I noticed a lot with my previous post on this subreddit and I'm really confused. Some of my comments kept bouncing around from being positive to negative in score but nobody was commenting or explaining anything that would make me understand why it would be downvoted. The post itself was also being continuously downvoted despite none of the comments seeming very critical of the question being asked. I know some things are obviously bad and nedd no explanation, but in cases like these which I see a lot across this website there isnt anything obviously bad so I want to know why there sre people downvoting on mass. Why does this happen?