How does a tool holder snap clean through like that?

That fracture face is telling you most of what you need. If it's rough and sparkly all the way across, with no smooth flat patch anywhere, it let go in one overload. Had it been cracked for a while first you'd see a darker, smoother zone with little rings in it before the rough part. Granular-looking steel like that is normal for a holder block, they're hardened hard on purpose so they stay rigid, which also means they don't bend much before they go. Shearing right thru the mounting hole makes sense too, the pin holes and cap screw bosses are always the weak spots. So yeah, that's either a crash or someone way overtorqued the cap screws. I've seen a guy split one clean around the screw hole by running a big breaker bar on it like the block was made of armor plate. If the threads on the missing screw are stretched or the head's rounded off, that's your answer. If the fracture runs across the body mid-block, more likely it got hit, spinning work or a wayward toolpost. As for the chip bin, I'd bet money someone crashed it and didn't want the conversation that comes with handing it in. Every shop's got a guy or two who figures buried under a week of swarf means it never happened. Doesn't necessarily mean the holder was garbage from the start. A perfectly good one dies the same way when someone forgets to clear the tailstock. Worth a quick look at your other blocks though. Grab a magnifier and check around the pin holes and screw bosses for fine cracks, and if you've got some layout dye or even a bit of kerosene, wipe it on and off and see if anything shows up sitting in the holes. Also check the cap screws themselves for stretched threads. If a batch of them came from the same source and one was soft or cracked inside, the others might be too.

And ask around the shop, casual like. Whoever did it is prolly relieved nobody's made a thing of it yet.

Counterpoint on the chip bin thing. Half the time it ain't a coverup, the broken piece just lands in the tray, gets swept in with the swarf during cleanup, and nobody notices till it hits the bin. Found a whole chuck key and half a boring bar in ours over the years. Didn't mean anyone was hiding anything, just lazy cleanup night. Still worth a wander around the shop though. If a holder let go mid-cut, there's ussually a witness. Scared-looking finish on whatever was being cut, or a nick in the ways somewhere.

Not buying the bit about granular steel being normal for a holder block. I've snapped plenty of these over the years and a proper one tears ragged and pulled-looking at the break, not grainy like coarse sandpaper. That sparkly coarse texture is usually a sign it got through hardened too far or tempered wrong, wich is exactly the batch problem the original poster is worried about. If one block came off the heat treat line bad, its siblings did too. I'd weigh the thing against a known good one of the same size before I blamed a crash for anything.

Agree with the cleanup theory, broken chunks end up in the bin more often than people think. Around here the catch tray under the lathe gets swept into a bucket every Friday and the bucket goes straight in the hopper, nobody picks through it. Lost a whole almost-new facing tool that way once, turned up three weeks later when we were cleaning the hopper screen. One thing to check on yours, if the missing piece of the block is still in the bin somewhere, the two broken faces togehter will show you which direction the overload came from, and that tells you crash versus overtorque without asking anybody a word.

Update: the grainy fracture face was the giveaway. Clean one-shot break, no smooth patch like you described, so it got hit with something hard rather than fatiguing. Also checked the catch tray under the machine and found the missing chunk wedged in the corner, so the swept-up-in-cleanup theory was right. Someone crashed it, it got swept, case closed.