Unfortunately, sometimes there is no particular reasons that we can think of, exactly like with humans, behavior and social interractions are not always rational.
If both the gliders are healthy and that there isnt a stressfull factor otherwise, it's just that they decided the colony had to split, possibly because there is a conflict over dominance which shifted.
you can keep on trying to re-introduce them untill they eventually settle down, while changing their environment, it might work at some point, but even if it does it doesnt imply that the same issue might not occure again.