I have a modest assortment (not systematic enough to be called a "collection") of SKSes and have owned about 50 different examples, all told, over the past 20-odd years. I've run into this phenom twice. One was truly memorable and embarassing with an entire 10-rd. mag rockin' and rollin' down range in one burst. Fortunately, the muzzle was correctly oriented and nothing was damaged but my calm.
Both times were with US-made commercial ammo and both carbines were surplus models, well-used, but in VG overall condition.
Neither case was due to a "stuck" FP. This was verified, as much as is possible, on field stripping after the incident. On both the FP moved freely when the bolt was inverted from the vertical 180 degrees.
Since I have never had this happen in any carbine while using surplus or true mil-spec ammo my conclusion is that the primers used in some US-made commercial generic ball and hunting ammo uses ( or at least did at the time) primers that are much more sensitive than military loads. I'm not sure whether this is due to the compound, cup material or a combination of both.
If you chamber a round in an SKS in the normal manner (pull back the charging handle to release and let the bolt run forward under the impetus of the recoil spring) and extract the round without firing you will almost always find a slight "dimple" in the primer cup.
IMO, the primers on military spec ammo designed specifically for use in semi, selective-fire and full auto arms with non-inertial FPs are made to be more resistant to impact forces than those generally found on "sporting" loads.
For myself, I limit my SKS shooting to mil-spec ammo. I don't handload for the cartridge as yet, as I also have a reasonable supply of suitable ammo remaining from happier times. If I had to use handloads or commercial sporting ammo in them, I'd put the inertial FP conversion mentioned above in my carbine(s). It's the only mechanical fix I know of that has proven reliable in avoiding the problem.