Since you looked for other signs of excessive pressure and found none, since the primers pushed in very easily, and since the primers only backed out of the Federal brass, I believe that you fell into a batch of soft brass (or brass with oversized primer pockets.)

A bad lot of brass is not unheard of. I had a box of 20 Remington 8x57 170 grain factory loads that I bought in 1981 or so. They were stored indoors under favorable conditions. When I bought a new Remington 700 CLassic 8x57 last year, I took these, plus a brand new box of the same cartridge purchased in November 2005, to the range to sight in. The 1981 lot was an unmitigated disaster. Out of 20 rounds, 12 rounds resulted in split case necks and accuracy on all of them was for poop. [:'(]The 2004 ammo was fine, and the new lot of Remington 8x57 brass I bought is also fine. I shoot ammo in milsurp guns that is from WWII with no problems such as this, so it is not the age of the cases, but the particular lot of brass, that is at fault.

This stuff would discard that batch of Federal cases and keep on going. It sounds to me like you know what you are doing in terms of looking for signs of excessive pressure.

Good luck!