I had a partial case head separation using a 1.8oz buffer

[DeNinny]

More photos. I was only able to find the slightest ridge forming on 2 cases. #9, and #36. #41 was the kaboom.

Assuming you didn't have significantly different loads in these rounds, these pictures indicate you were pushing the pressure limits of this brass...with all the other issues in effect as well...deep feed cone, deep bolt face, headspace, etc. You were close to the failure limit, but not in massive excess of it. So only a few rounds instead of all of them deformed beyond the norm.

Also the pics still keep heat on the table too. As you were shooting, the barrel was heating up which was heating up the rounds and the starting temperature of the combustion. Towards the end of your batch the barrel would be warmer, so your risk of failure kept going up.