I saw a M1-A break the exact same way minus the OOB kaboom. It was at a NRA Highpower tournament the weekend before the 1994 AWB was set to begin. I think he sent the rifle overnight to Springfield Armory for replacement.
The heel of M1 Garand and M14 based receivers absorb an impact every time they are fired. Normally they rarely break. I have one M1-A that hammers the tang on the firing pin. I don't know why, it just does.
Hit on something long enough and things can break.