Back in my Reddit days I got banned twice - once from a subreddit and once a site-wide shadowban (which got rescinded when I appealed, amazingly) and both were random bolts from the blue where I didn't actually break the rule I was banned for. In the shadowban case I happened to belong to a subreddit that was apparently in the midst of brigading another subreddit I was reading, and when I upvoted a few comments I guess I triggered some kind of anti-brigade filter. In the case of the subreddit ban, there was a guy being downvoted who was complaining about it and I explained to him why I thought it was likely that he was getting downvotes. That subreddit had a "no downvotes allowed" rule and the mods must have figured that since I was explaining why the guy was getting downvotes I must also be downvoting him. On Reddit there's no way to actually tell who's downvoting who.
Here on the Fediverse it's both a bit more scary since every instance can have whatever standards of care it wants, but it's also less scary because every instance can have whatever standards of care it wants. I can just create a new account at a different instance if the one I'm on turns out to be run by yo-yos. Hopefully the account migration features get implemented soon to make that even easier.