Why does marijuana buds get seeds?

Female marijuana plants will seed for several different reasons. Number one is, your female plant was pollinated at some point by a flowering male. either by the wind or an insect etc. another reason may be that it has become hermaphroditic which means that it can pollinate itself to produce seed. this caused by many different things. Stress is a big cause, heat, light, water, air, temp. all of these things play a factor for decided the sex of a the plant. if there are any males in the garden get rid of them safely before they mess up the whole thing!