There's a theory on the brain, named 'predictive coding'. I'm not a neuroscientist myself, nor a specialist on A.I. (the theory is also being used in the field of A.I.) and it's a very elaborate theory, so i don't know if i'm going to do the theory any justice with my layman's explanation, but i'm gonna try: The theory basically states that 'perceiving, is predicting'. What the brain is doing, according to this theory, is making predictions, based on the information available. But here's the thing: It's not directly making predictions about the world outside. It's making these predictions indirectly, because it is actually making predictions about itself. It's making predictions on how it's going to respond to all that's being thrown at it. And it's doing so layer by layer...OK, that sounds a bit mystical perhaps, but i just mean that the theory states that the brain is a very complex, multi-layered structure, and each layer is making predictions on the layer just below it.
The crucial point is: innevitably, as the brain is not omniscient, mistakes are being made. The brain needs some kind of feedback. I believe they call it 'error-signaling'. And it's very crucial ofcourse. Error-signaling is important because it helps the brain to learn from mistakes, but more importantly, it prevents the brain to sort of get lost within it's own little world. It's our contact with the world outside.
It's easy to see how hallucinogens fit in here. What hallucinogens seem to do, is to temporarily block this very crucial mechanism of error-signalling.
But if the mechanism is blocked for too long, then what happens is that the brain is no longer being corrected. Contact with the outside world is being cut-off. So some people become compulsive. They constantly have to wash their hands or something, because they no longer get the signal "hey, you there...your hands are fine, now get on with your life". The brain keeps making the same predictions over and over again, going round in circles, without being corrected. People start to ruminate, or become over-anxious.
Or at least, that's how i interpret this thing.