Why does my Betta fish sometimes freak out...?

[Peanuts]

No one would be around and I'd be across the room and here this tapping on glass, and find out that it's my Betta fish. He keeps literally just flipping out. He taps the glass of his fish bowl really hard, looks back and forth, and his flippers flip a million miles an hour, and no one is even there. It's a pretty big bowl, but he hangs out in the same spots. Is there anything wrong with him?

 

[catx]

Its definitely a reflection he can see. As far as he's concerned there's this other fish flapping his fins and flaring at him, but this strange invisible barrier is preventing him from going in for the kill and defending his territory, he just can't work it out!