Are you sure it was previously deleted stuff? I thought the same thing had happened to me but it was due to subreddits being private at the time of deletion then later coming out of private (some weeks or months later) preventing those then privated posts/comments from being deleted. I think running another automated tool again should do the trick at this point.
Any popular posts that involve a minority/enemy of right wingers doing something bad or sticking out get brigaded. A blatant example is PublicFreakout where threads are usually fairly normal unless it’s a black/arab/Indian person doing the antagonizing then pretty much all the top comments are dog or regular whistles. Similar “brigading” can happen even in a city subreddit similar to r/Canada even if they are regular users otherwise. If the post is good enough fodder the subreddit will suddenly resemble a klan meetup even if it’s usually otherwise “normal”.
ActualPublicFreakout is an alternative that doesn’t need brigading because it’s already similar to WorldNews.