With a good chamber, you are doing pretty good at eliminating possible causes.
This really does not line up with just a larger than bore bullet. Maybe there was something going on with the larger bullet and the new brass. You did have problems pulling a bullet from a loaded round.
I have one other that sounds unlikely. If the round was near max with some space above the powder, it is possible for the bullet to have a glancing blow on a edge on the way in and get pushed deeper into the case. This will push pressure up, but probably not as much as the case indicated. Also, this seems real unlikely based on the hard to pull other round.
Where are we now on "not eliminated" causes. I can only swag 2, are there any others still worth talking about?
- Having a film on the ID of a new case combined with a real tight fitting bullet such that the bullet is "glued" in place firmly enough to cause a spike.
- A very strange primer/powder interaction with a secondary pressure spike (very low probability)