I often wonder when I see these:
1) How much force was required to install the block on the barrel originally
2) How rapid were the shots preceding the failure
3) What material is the gas block made from
If 1) is "lots," it is highly likely the heat from 2) expanded the barrel just enough to cause fracture, particularly if 3) was a brittle aluminum alloy like 7075 hardened. The location of the failure suggests gas pressure had nothing to do with this, and even if it was, it would only act on a tiny area the size of the gas port (and if that pressure isn't enough to rupture your gas tube and bolt carrier, it won't pop the gas block). If somehow you had a void between the gas block and barrel, pressure could act over that, but you'd also get tons of gas leaking around the block, which would keep pressures below failure levels.
I know most folks don't build ARs and therefore would not know the force taken to install the gas block. I have a suspicion that manufacturers are press-fitting them on thinking they are more stable, or something, when set screws or a pin are more than ample. Since the block is a thin continuous ring of metal, forcing it over a larger diameter raises stresses real fast, and will cause outright failure or early fatigue/corrosion. If the object it is pressed over expands as it gets hot (like a gun barrel...) that extra couple thousandths of an inch may push it over the edge.
FWIW, the gas block on my AR70 is a pretty loose fit, retained by the muzzle deviced screwed down onto it, so I doubt gas getting between the block and barrel could cause this. My money's on a material imperfection (you'll see a dark spot on the crack surface) combined with excessive tension caused by the block being under tension around an oversize barrel (or undersized block). Also check to see if there are multiple "waves" or colors in the crack; each of those will indicate how far the crack jumped after each shot as it fatigued out on you. That the block "sprung open" and slid around like that after cracking strongly implies it was under a crap ton of tension.
TCB
