Speculation, short stroke resizing leaving area near the case head too large. This area was jammed into the chamber from first and repeated unsuccessful attempts to close action. Might be a tight fit considering the OP says they were shooting multiple rifles chambered in 300Sav. From this I have to assume reloads were also from a variety of rifles with slightly different chambers.
The round being lodged so tight in the chamber would require some sharp blows from the muzzle end to free. If the round was still held by the bolt inertia from the blows may have pushed the firing pin spring back just enough to allow it to come forward and ignite the primer. The blows may have also pushed the primer out of the pocket slightly resulting in a close relationship between bolt face and primer.
Pure speculation. Just an idea. I’ve recently encountered similar feeding issues neck sizing brass from the same rifle. Bolt hangs up close to getting into battery. Difficult to reverse and get round back out. Haven’t had to drive any out from the muzzle but they where tight! I’m FL sizing for the rifle now.
Very interesting thread for me, I’ve learned from reading through what has been presented.
