I'd suggest it could have something to do with the fact that you have two female bettas in there. Normally two bettas don't get along - you need stable groups of four or more to balance things out. It could be that they go angry and took it out on the glofish.
It could also be that glofish, which are nippy when not kept in schools of 6 or more, especially in smaller tanks (they do much better in 2ft tanks, where they have lots of room for all that danio energy), tried to nibble a betta and got attacked in turn.
A third, and equally likely scenario, is that one of the betta's just decided to prove that bettas are crazy, unpredictable things and went on a killing spree.
Given that you have a ghost shrimp and two carnivores in there, it's also possible the glofish died of something unrelated and got eaten by his tankmates. When I lose a fish, if I don't find the body within 12 hours, I will never see it again because my cories, shrimp and betta girls will destroy the evidence.
You do have a risky set-up there, and sadly that does result in deaths sometimes. :(