if no light leaks or added stress, it's genetics... too much inbreeding or a lazy selection process by the breeder etc.
nanners aren't so bad on their own. They don't produce a ton of pollen and somtimes it's not even viable pollen. If you see a pollen sack, it's time to shit your pants, lol.
Pluck them before they mature and they can't do much (green colored). Yellow means it potentially produced some pollen and released it.
Happening 1 time ... shit happens. if it happens frequently, either a shit ass breeder or your environment is causing stress.