Strictly conjecture on my part, but I'd guess "handloaded ammo with a high primer leading to a slamfire".
It's my understanding that early in the life of Garands one was tested with a custom load which was estimated to produce 120,000 psi, roughly twice that of a standard load (probably fired with the gas-cylinder plug removed).
As the story goes, it broke off one of the locking lugs on the bolt, but no other damage. It went on to fire another 5,000 rounds (with the one-lugged bolt) uneventfully.
Factory Wincheester ammo and a brand new Garand. Training by a former Marine.