This is from the game manual, section "Ground conditions, bogging and immobilization":
When issuing Movement commands, keep in mind the ground condition that you
want to order a unit to move over. All vehicles are rated for Offroad performance.
To some degree better quality crews lessen the chance of bogging. However, if
you order a non-tracked personnel carrier to move across a muddy field the best
crew in the world won’t likely help you out much.
[...]
Bogged vehicles display a “Bogged” marker in their unit panel, and are not able to
move. Bogged units attempt to un-bog themselves automatically. This may fail,
however, and lead to a permanent immobilization.
According to the manual bogging is related to ground conditions and not mechanical failures.