This is another reason why a lack of transparency with user votes is bad.
As to why it is seemingly done randomly in reddit, it is to decrease your global karma score to make you less influential and to discourage you from making new comments. You probably pissed off someone’s troll farm in what they considered an influential subreddit. It might also interest you that reddit was explicitly named as part of a Russian influence effort here: www.justice.gov/opa/media/1366201/dl - maybe some day we will see something similar for other obvious troll farms operating in Reddit.