The pecking order. When kept in a community tank, bettas will establish a pecking order. One dominant fish will boss everyone else. As long as that fish is in the tank, there will be pretty much, peace. Remove that fish, or add new bettas to the tank, and the pecking order has to be established all over again.
Fights arise, bettas fins are frayed, and for a week or so, the whole fragile balance is threatened.
Eventually, it settles down, and peace returns. Hey, I fooled you. Of course not. How bettas survive in the wild is also a puzzle, knowing how unsociable they are. After all, I heard a researcher took two male bettas and went to the middle of a large lake.