Is it possible that the Federal case neck was long enough to protrude into the forcing cone, squeezing and holding the bullet long enough so that excess pressure was produced before it was released?

How about the bullet being jammed into the forcing cone as the round was chambered,once again producing excess pressure on firing?

Measurements of the new Federal cases and bullets might be interesting, but might not show the same dimensions as the "problem" round.

The only other possibility that comes to mind is too much powder, or the wrong type.