Just my logical thinking, they can be in a school in the wild as there are vast space and big ocean with natural predator. Schooling is a natural behavior for some fishes as it protects them from their prey.
However, when they are captive in a tank, space becomes restricted, there are no longer natural predator present, and hence slowly every individual fish will try to territorise their own territory. Thus, the end result will be fighting among themselves until the number of fishes where space is sufficient for.
Make sense? This is just my obervation from watching Discover Channels and Animal Planets.