cannabis seeds have a strange reaction to stress.

if they feel that the conditions are bad enough that they may not be able to reproduce, they'll switch sex in order to self-pollinate.

are you sure some of them weren't hermaphrodites?

if you do a little searching, you'll find that even feminized seeds can switch sides if conditions are bad enough
little things like improper watering, ph fluctuations, & temperature swings can make it happen

100% feminized only means that the seeds were bred with female pollen - that in turn means that the seeds have a higher likelihood of being female
the best breeders test for stability meaning that the seeds can stand up to a tremendous amount of stress without turning