Stress causes herms.alot of factors can do it temp,nutes,bugs poor plant health.2 week into bloom look at the lower not the upper nods thats how they sneak you.you will see the pollin sack.If it goes herm late in bloom you will see yellow bananna like sacks in your buds.that is female pollin.if you wish to collect it.sometimes you can have the perfect garden and get a bad pheno it happens.guess the key is watch your garden.