IIRC (it was almost 30 years ago), the trigger return spring was hanging up on a rough/poorly machined surface. If you pulled the trigger part way, then decided not to fire, the spring didn’t have enough power to return the trigger to the normal static ‘at rest’ position, and cycling the safety caused sufficient movement in the mechanism to (under certain circumstances) release the hammer. Smoothing out the machine marks allowed the trigger & trigger return spring to function as designed, and fixed the issue with that particular firearm...
Thanks. I always found the SKS trigger group to unnecessarily complicated. I laughed out loud the first time I saw an AK trigger group.