What the fuck was this?

Was at a bus stop. Some girl showed up. She heard me talking on the phone. Asked when the bus was do. I told her. We started engaging in light conversation. Subtle flirt energy. She says half serious-half joking that I should give her my bus pass so she can ride. I sarcastically say totally. Then get on the bus and walk to my seat pretty far in the back. She immediately walks right towards me and sits directly behind me while saying “You shoulda gave me your bus pass”. I figure she was still joking so I just say “Totally” again.

2 minutes of silence go by. I’m scrolling on my phone. And then, and I shit you not, she literally just smacks the back of my head. I immediately turn around and say “Look at me. Look at me right now. Do not do that.”. She doesn’t even make eye contact and says “Who did that? Who did that?”. I immediately calm down before I go off and say “It was very nice talking to you. Have a nice day” then just walk to the front of the bus and act like I never met her.

What in the fuck possesses a person to act like that?