Angelfish neck nipping

[Keatonm]

This is my first post! I have a 75 gallon fish tank that is extremely planted (almost overgrown) and five angels and other peaceful fish. One of my marble angelfish is nipping the other one on its "neck" why is that? Also the other marble comes back for more. They are both a little smaller than dollar coin size.
The other fish get chaises around sometimes but rarely. What is happening?

 

[Delapool]

Mostly I find it is keeping the pecking order but not really truly aggressive. I watched them back off from doing this if the other one is distracted most times (very gentlemanly I guess), on the other hand one lost an eye.