Someone please explain this to me

[RedDevil]

possibly some modifiers in play here would be

minus:
for crossing open ground
Taking fire in rear/flank
HQs unit (bullet magnet status)
Suppressing fire from a MG (MGs are big time suppressors)
natural Soviet unit fragility

Possible:
Taking Fire from another source not spotted due to cautious stance prior

So there's quite a possibility that you had a LOT of negative modifiers to your boys' chances of making it across that open ground and they failed the morale checks because of them.
There's a good chance they failed multiple checks, and thus incurred multiple drops in status leaving you at the broken stage.

They could have routed and began retreating across that open ground and then started losing men from covering fire.

you're better off they're broken, as a few minutes in cover without being fired upon would prolly bring them back to fight again, whereas routed would have them running all over the place taking losses and not be worth a poop later on.