If the seeds are mature, you may have had pollen flying around your growing area during flowering. This can happen if you have a plant that has made male flowers that you didn't notice or on a previous crop and didn't completely clean your tent before moving on to the next crop.
If the seeds are really small, this may be a natural reaction by the plant, which is so desperate to reproduce that it forms male flowers in the buds.
Anyway, you should always watch your plants for male flowers throughout the flowering period, so that you can remove them as quickly as possible. Under the branches at the beginning, in the buds at the end.