Your primer blew out because of a loose primer pocket and had nothing to do with a light load, a gas operated rifle can have pressure in the barrel as the bolt unlocks and moves to the rear. This pressure can force the primer out of the primer pocket and also move the shoulder of the case forward and the case can end up longer than the chamber.
The gas port on the M1 Garand is at the end of the barrel and the bullet leaves the barrel before the bolt begins to move and has no pressure in the bore. A M14, AR10 or AR15 has the gas port at the mid point of the barrel and pressure is still in the barrel as the bolt begins to move to the rear. So again a loose primer pocket and residual pressure in the bore caused the primer to "pop out" under pressure.
Bottom line, there is a reason why military primers are crimped in place.
Below is a animated image of a cartridge being fired in a bolt action rifle with a stationary bolt, as the pressure builds it forces the primer out of the primer pocket until it contacts the bolt face. As the chamber pressure continues to build the brass stretches to meet the bolt face, the amount the primer can back out normally is equal to your head clearance or the air space between the bolt face and the rear of the case.
Your head clearance is the same as the amount of shoulder bump your reloaded case has, the problem with a gas operated rifle is the location of the gas port and the residual pressure in the bore as the bolt begins to move to the rear causing the primer to move even further to the rear. And if the primer pocket is loose this residual pressure will force the primer out of the primer pocket and it normally ends up binding up the trigger group.
I use pin gauges to check for over sized primer pockets.
And if I can push the new seated primer out of the primer pocket using a Lee depriming tool with just finger pressure the case goes in the scrap brass bucket.
What happened was very simple, the bolt unlocked while pressure was still in the barrel and the case was still gripping the chamber walls. With the residual pressure in the barrel and the case gripping the chamber walls this pressure kept pushing on the primer and the bolt causing the primer to "pop" out of the primer pocket.
The powder burn rate and the location of the gas port effects the residual pressure in the bore and a crimped or "TIGHT" primer pocket keeps the primer from popping out of the primer pocket. (it one of those Newtons third law thingies)