YouTube demonetizes videos left and right, often for no rhyme or reason. (In fact, some people have tested what happens if they upload the exact same video on their respective channels, and have confirmed that sometimes one of them will get demonetized while the other doesn't, which goes to show that demonetization is not always about the content of the video itself, but something else.)
Likewise YouTube seems to love penalizing channels for "community guideline violations", and distributes penalties like candy. Usually if a channel gets three such strikes, it's terminated automatically. Sometimes it's terminated sooner.
YouTubers who experience this always seem to have the same issues with the system:
Firstly, usually YouTube doesn't give the channel owner a chance to correct the problem before his channel gets taken down. There is no "you have 24 hours to remove the offending content or else" messages, or anything of the sort.
Secondly, and more importantly, YouTube never seems to specify exact what was the infringing content. Sometimes the individual infringing video might be mentioned, but not what exactly is the infringing part, or exactly which rule is being infringed. Othertimes not even the infringing video is mentioned in the (usually automated) message sent to the channel owner by YouTube.
This can be really aggravating to YouTubers. These messages never specify exactly which rule was infringed, ie. what the exact reason for the strike or termination was. The messages are always extremely generic, typically only mentioning "community guideline violations" without going into any more specifics.
Bigger YouTubers, usually those who have contacts within Google, or know influential people who have such contacts, may get the actual reason for the strike from them. Their contacts will dig the YouTube's systems and find out what exactly was the reason. Smaller YouTubers, who have no such contacts, might get the actual reason once in a blue moon when they appeal the strike, but that's just pretty much up to chance.
The problem with this is that when YouTubers don't know what exact kind of material infringes the rules, how exactly are they supposed to know how to fix the problem, and to avoid the problem in the future? The striking and termination systems are largely automated, but there seems to be no automation that would reverse these penalties once the infringing content has been removed or fixed, especially since the channel owner is given no information on what is the infringing content.
This is doubly true with video demonetization. YouTube never, ever reveals the reasons why a video gets demonetized. It's effectively a well-guarded secret. Not even those contacts within Google can tell you why a particular video is demonetized, or what could be done to avoid it. And that's the problem: YouTubers have no way of knowing what kind of content to avoid, or how to make their videos in such a manner that it won't get demonetized. Google gives exactly zero information about this. (For all we know, not even they know, perhaps because the decisions are made by a self-taught neural network, which thus, for all intents and purposes, demonetizes videos unpredictably and almost at random. For all we know, there is no explicit list of things that will cause a video to be demonetized.)