The reality is that 2 years is not long term for a species which lives decades. At about the age of 2 years is when clowns really start asserting themselves. Additionally, there may be other factors like stress which are playing a role.
In the wild, clowns do live in unsexed groups with a single lead female and secondary male. Like any species, however, individuals want to climb the social tree and get a chance to reproduce. As males and females die, the social order shifts. Otherwise, eventually individuals will challenge the male to gain access to reproduction.
In the wild, the losing fish swims off and joins another group (or gets eaten, if he/it doesn't do so fast enough). In captivity, the loser fish has nowhere to go to and the fight will become deadly either by outright killing or the loser fish will jump to escape.
Fish stores get away with keeping clowns in groups for short terms because the fish are too stressed to fight over hierarchy.
Breeders get away with keeping juvenile clowns in groups because they are so young and because you pack them in gill to gill, which reduces aggression (no one has a chance to establish a territory) but also necessitates daily water changes and makes an environment unhealthy to corals and anemones.
Even so, eventually the flock thins. I had to artificially reduce the size of the tank (using mesh) to cram the fish closer in together until they stopped fighting. This is with the gentle A. occelaris.
Maroons in particular are difficult for breeders and juveniles will kill each other in grow-out.
I have seen multiple pairs of the more docile species kept successfully in very large tanks with specific aquascaping plans. But we are talking typically tanks in the 200+ range where the clowns can be visually separated from each other.
