It was a a wild hunch but I'm not sure that's what it is.
They don't tolerate others of their own kind very well unless it's their own mate. I had 2 females that tolerated each other in a 120 for a while but eventually the older one took out the younger one - they were CB, biological sisters from different broods, about 2 years apart. Before that the older sister had a mate (her brother...) but he died after a Flatworm eXit problem, and I tried to match her up with another and it didn't end well.
It's nearly impossible to sex them except when the male is holding eggs and his jaw will become much more square and pronounced.
Juveniles stick together for safety but once they mature, they don't want anything to do with any other Banggai except their mate.
There have been lots of issues with wild caught specimens having some sort of infection or infestation that kills them off. I'm not sure if it has been identified or not, but what is an otherwise very hardy species, has had lousy survival statistics from wild caught specimens in the last 10 years or so.
Jenn