Stalls are unnature for a horse. They know the field and the open space. Being trapped in a box is bad in the eyes of an horse that isen't accustom to it. There is no room for flight and that is a horses #1 concern. If something is going to eat me where am I going to go, what are my options, how do I get out of here. I do the same thing I don't know why but every time I go somewhere I quick survey, where are the exits?, fire extinguisher/alarms, light switch?, who's in the room, if someone walks in with a gun what can I pick up as a weapon?, Where will I hide? It's survival instincts that some people/horses have a stronger tie to. Everyone probably has it to some extent, but some more than others. I'm not saying that it's impossible to fix the behavior. He needs to feel that nothing is going to try to eat him. Safe. Even once you get him to stop banging at the gate I am sure he will probably be a stall walker.