Betta's like most of natures creatures, when living in a community structure will eventually form some sort of controlled structure betwen the fish. One dominant fish, usually a large, healthy male or sometimes female will boss the other fish around. Just like with gorillas they are known as the 'ALHPA' fish. There will be some madness between the fish and a lot of pecking until the ALPHA position has been established. However, once the ALPHA has been established there will actually be peace in the tank.
Remove the ALPHA or add new fish and the cycle will start again as a new balance must be created.
It's natures way.