I unfollowed some accounts after days & weeks, when they posted way too much boring stuff. Some people post something every couple of minutes...

When they however immediately unfollow, right when you follow back, then this sounds like this scumbag tactic, mostly used by malicious bots, which farm follows for weeks or months & then post ads/scams/propaganda & justify their account's existinece through the amount of followers.

I would recommend to only follow if you have a legitimate interest in that account's content.