There are reasons to downvote, most of it seems to do with greed. Some people do not want to see other post get a big payout, big payout being a relative term to the one doing the down voting. Some people think they can downvote anything for any reason at all. Then there is the "do as I say, not as I do crowd", that will down vote anything at all they do not like.  It does not matter where you go in the world real or virtual, there are always those that want to control what you can say, or do. Only thing I can say is if it is a meaningless abusive down vote, report it to @steemcleaners, they may do something about it, they may not.

The number of meaningless "social Justice warrior" down voters are beginning to fade away. They have become meaningless to most of the people on steemit. Most of the down votes are at only $0.0001 completely meaningless, laughable in fact in most cases. Those down voters are so afraid of life and reactions to their down votes that they are very careful to have never uttered a word on the steem blockchain. 

Some people like to dish it out, they just can not take it when it is served back up to them. Ignore them, or report them, only two real options. When/if you ever get high enough on the reputation chain to where their retaliatory down votes can not touch you then you could think about stooping to their low ego/self esteem level and down vote them as revenge. but it would get you nothing.

Can't really add much to what @bashadow said... the "random downvoters" are just some disgruntled people who got extensively flagged for plagiarism, spam and shitposting. The particular downvote I am thinking about (I get it, from time to time) is all but worthless... it's something like a 2% downvote from an account with 50 Steem Power and perpetually 5% remaining voting power. 

It's effect is meant to be purely *psychological,* to make newcomers feel unwelcome and thereby "hurt" Steemit. I have learned to just ignore them...

=^..^=