Someone please explain this to me

[The Coil]

Wouldn't shaken or pinned be more likely?

I've never seen a unit break THAT fast before in CM.

More likely? Maybe. But 'more likely' doesn't get you far when you apply it to a specific situation. Maybe there was a 99% chance of getting pinned and a 1% chance of breaking. Maybe this is an outlier result. I know I have a tendency to always want the expected result to happen, and to feel cheated if it doesn't (unless it goes in my favor, in which case I'm a brilliant tactician). My Tiger kills a Stuart, all is well. Stuart kills my Tiger, the game is broken. My Stuart kills a Tiger, I'm a great CM player.

Is it within the realm of probability that an already jittery regular unit hit in the flank/back with MG fire would break. Sure. If it happens every time, maybe that's excessive. But you seem to be pointing this out as remarkable and out of context with the rest of your CM experience. Accept it as that. Sometimes wacky things happen. When wacky things happen regularly, the game is broken. When wacky things happen every once in a while (as this seems to be), that's a good model of reality. :soap:

That all being said, my best guess as to what happened is that the unit took fire from something you never saw. Given that it is an HQ unit (sniper magnet) in the open, it probably got sniped. A sniper will panic/break a unit in the open with one shot, esp. in conjunction with MG fire and an already heightened morale state.