If there was residual powder in the barrel from previous shooting .. one shot one week earlier... would a spark created by the barrel hitting the ground travel up the barrel via the residual powder and ignite the two 777 pellots causing the gun to fire... I mean is that possible? Barrel hits rock.. rock creates spark.. spark travels length of barrel to pellot charges and ignites pellots causing the gun to discharge... What do you think?
I don't see how a spark could travel up almost 2 feet of barrel, past a presumably tightly fitting bullet to get to the powder charge... Assuming it did, it takes quite a spark to ignite BP substitute pellets, that's why your rifle uses a 209 primer- because they spit out a heck lot of lot of flame compared to a #11 percussion cap thereby ensuring positive ignition with BP substitutes.
Now, I'm just speculating but... the priming compound of regular pistol / rifle primers is supposedly very, very easy to ignite, I imagine a 209 primer is similar. Normally primers are fairly safe because, I think, the priming compound is sort of glued together (otherwise it's a fine dust). If you accidentally, but gently, decap a live primer you're probably safe, but you'll crush the primer anvil against the priming compound and probably leave a trail of the fine priming compound- somewhere- that's the dangerous part (I've heard). Supposedly it doesn't take much to ignite the dust. I've accidentally decaped a few live primers in my day but I've never tried to ignite the stuff, so I can't be sure... I'm just repeating what I've heard.
Having said all that... Unless you were smoking a cigarette and some of the "cherry" fell off and landed on the primer which heated the priming compound (I believe you said the primer was exposed), or unless some other "thing" smacked the primer while your rifle was free-falling towards earth and if, indeed, the bolt was back, and did
not travel forward somehow- I guess, it's possible that your 209 primer could have ruptured somehow during loading and maybe some of the priming compound's dust got ignited, perhaps, by the primer itself under it's own weight- I don't know.
...Honestly, these ideas seems a bit far fetched but it's all I can come up with. I don't believe the pellets could have ignited from any sort of impact or from some errand spark at the opposite end of the barrel while being shielded by a bullet. The ignition had to come from the primer, somehow. Primers are, after all, and by design, percussion sensitive.
If I had had this happen to me, I'd call Knight post-hast and get their take on things, describing everything you have here.
Good luck!