Yes, it does happen sometimes. Someone will go to a person’s profile page or community’s page and downvote everything. Occasionally they will use multiple accounts. Very occasionally they will even script it.

We consider it to be spam and we ban them.

Occasionally someone will use alt accounts to upvote themselves. We treat those the same way.

There are people who will act out by down voting everything you post if you upset them. Possibly the person making the innocent posts responded to one of the down voters previous comments in a way they didn’t like.

I’ve begun to suspect that someone is running bots to do it for some unknown reason

Looks like you got one! Too bad they didn’t explain why. Could just be a joke here due to the context.