Any consideration on ammo... Reloads.. wrong powder, like an oops of pistol powder or over charge. One off mystery load? Squib in the barrel? Takes a crap load of force and pressure to rip metal of any thickness. Even something as simple as excess oil in the barrel could build high pressures.
X39 ain't a super high pressure round compared to say a .30-06, 8mm or a full blown cartridge, something else happened I think.
If this 1 receiver was weak.. how many others hold up, it could be a 1 in a million failure, but there are a metric crap ton out there that have no issues and people hammer the hell out of them.