I have seen this happen with a charge that was WAY TOO LIGHT but it doesn't sound like the op did that? What happens is the powder moves around in the case leaving large air pockets which can cause strange things during ignition. I had this happen with a very light charge of fast burning powder many years ago. I was shooting 75 grain bullets in a 25-06 at a little over 2000 fps with no problems when one of the rounds just made a hisst sound and I felt gas on my face! When I grabbed the bolt, it felt like it was welded. I had to hammer it open with much difficulty! When I finally did get it open the brass was fused into the bolt face so badly that I had to pry it out with a pair of vice grips. You could have put a shotgun primer in the pocket, it had a belt as big as a WM, and had a brass knob that had protruded into the ejector. This round didn't even go bang, just hissed!
Now, even when I use powders like 4198 for greatly reduced loads, I make sure the powder is next to the primer!!
I don't that this has much to do with the it's experience, but merely point this out to show that it isn't always an OVER CHARGE that blows things up!....rich