Many of the SKS sold were military surplus from the various source countries and were usually covered in heavy preservative grease. The grease got into ever nook and cranny (as it was supposed to,) and could prevent the firing pin from moving freely. If you didn't disassemble and clean the bolt and make sure the firing pin could move freely a slam-fire would be possible if the firing pin was stuck sticking out of the bolt face. The other way to cause a slam-fire is if the hammer catch is worn so the hammer isn't caught and held. What happens then is the hammer follows the bolt as the bolt returns to battery and if a new round was chambered the hammer, pressing on the back of the firing pin and forcing the pin to stick out from the bolt face can cause the round to fire. Either way it's bad because both problems can cause the round to fire before the bolt is completely in battery (fully forward with the lugs or locking mechanism engaged and the round fully supported by the chamber.) That can lead to the rear of the cartridge rupturing and venting burning propellant, usually into the magazine.