Damn man, that doesn't sound good.
As far as I know there are 3 ways a plant will herm.
Bad genetics, stress or a light leak.
Seeing as your plants look happy and it is multiple strains, I would lean toward there being a light leak in your tent.
Switch your grow lights off and the lights in the room your tent is in on and close yourself in the tent. If you see any light coming in, block it.