Stress to the plants is the usual cause but it can also have to do with the strain. What strain are you growing? Are you growing from seed or clone? If seeds where did you get them? Are they feminized? Bag seed?
Hydro or soil? Type of lightning? Temp? Humidity?
EDIT: I had a plant where just one branch turned on me, took the branch off no problems after.
I'd take herm plants and set them outside, sterilize the room and get back to growing. Come back and harvest the hermies later, it's still bud after all, and hermaphrodite seeds tend to be female more often than not.
are you pulling all your plants that go hermie on you from a mother in veg ?? just curious because if its been stressed and gone hermie on you then no matter what you do when you flower the clones they aill be herms . and because you have the mum in veg it wont be showin her balls.
its a bad boat to be in havin yer mum go hermie on you cuz it leaves you stuck killin off yer genetics and gettin a fresh start