While maybe not exactly a bug, this is an abusable glitch.
Someone used his kubel to push my infantry out of a trench so it didn't have cover to counter the Kubel.
Basically what seems to be happening is, that the infantry unit in the trench does the usual 'dance' to avoid a vehicle crush.
However, in the case of the trench, it means leaving it all together because some of the models do and then the whole unit rejoins outside the trench.
Infantry should not leave a trench to avoid vehicle crush sinde it cannot be done there. They also shouldn't do this 'dance' versus vehicles like the Kubel that don't do the crush anyway.