CS, STS, GA3, Aspirin, willow tea, etc. Stress from abnormal lighting times/RH/temps, stress from air injection, rodelization (I have never had this work ever, for me the plant just dies) etc..
Natural hermies do carry a chance of making hermies, as will the seed from the pollen donor (S1).
CS is the most used as it is the most reliable, do not use the buds that were sprayed, toss 'em and only keep the seed. Do your homework on using it, once done I toss everything that touched the CS in the trash. The soil, the pot, the sprayed bud, the spray bottle, everything is tossed.
One can make their own CS, it is not hard or costly.