Imperial Guard troopers have sandbags around them whenever they crouch

[Marrik]

Why is it that any time one of my guardsmen crouches, a little stack of sandbags magically appears in front of them?

[Leflair]

That would be the DIG IN ability they have.
An earlier version had a visual bug with it where it would show the sandbags regardless of the ability being active or not (a button up to the right side of the screen).

DIG IN grants a 50% cover bonus (works like dodge, nullifies a hit), each successive hit reduces the bonus by 10% until it is nullified. IIRC should only work vs ranged attacks (and not explosions/melee).