Why is he acting like this?

This is from a manager. He is nice to me sometimes but also mean. I used to ask him lots of questions but now I feel a little uncomfortable. Slowly he has started touching me more often on my back and lower back and likes to stare at me from across the room. I figured he was being protective. On other days he is mean and likes to humiliate me in front of co-workers if I make a mistake or ask him a question. Because of this, I have started to distance myself. He confronts me the other day accusing me of "not wanting to be his friend" since I try to avoid him as much as possible. It is really sad because I think he is approaching his thirties. What is going on with this man.
Why is he acting like this?
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