One common reason is that the pan has been scratched with a sharp metal tool, such as a spatula. Sometimes the cause is indirect. If one lets a pan rust, it probably needs to be washed and very likely scoured with soap or detergent. When cleaned in this way, some of the oil that coats the pores and minuscule jagged peaks of the metal bind themselves chemically to some of the cleansing agent's molecules and flow clown the drain with the dishwater.
Naturally, the more fiercely one scrubs, the stronger the cleansing solution, and the longer the pan is soaked, the more the pan becomes deseasoned. If damage done by the cleaning is not too great, the pan will automatically reseason itself the next time you fry in it — so no harm done. If the damage is severe, you will have to start the seasoning process over again from the very beginning. If rust has developed deep inside the pores of the interior surface, dump the pan into a trash bin as it is not serviceable again.