Well, first it is NOT a sensitive primer going off from the bolt hitting it. The primer shows a clear firing pin mark. Nor do I think it is a result of firing pn creep, where the inertia of the firing pin causes it to impact the primer. When that happens the primer is usually flattened back out since there is no resistance to the firing pin being forced backward.
The firing pin mark is too clear for that and very obviously the result of a full hammer impact on the firing pin. (That type of firing pin "crater" requires a certain firing pin support and the primer clearly shows that support.)
Further, the firing pin mark is centered, meaning that the cartridge was fully into the bolt face and under the extractor so the bolt had to be around 1" or less from complete closure.
I think that at one point, the bolt or the carrier stopped its forward motion, short of battery, for some reason. The shooter, not realizing the bolt was not fully closed, pulled the trigger. (That gun does not have a disconnector.) Some defect of the gun allowed the hammer to reach the firing pin; the extractor held the case head firmly enough to ensure good support for the case and permit a full firing pin blow. And ka-boom!
The case seems to confirm the above theorizing. The case blew out, but enough was in the chamber that high pressure gas came back around the case, crushing it. The internal pressure meanwhile had blown the head off the case in the classic "excess headspace" condition. There was probably enough pressure to push the bullet out the barrel; the OP doesn't say, but I think he would have mentioned a stuck bullet.
There you have MHO.