Some vehicles stop moving and take damage by themselves when heavily damaged. The idea is that if a vehicle is nearly destroyed, it is probably on fire and will soon be fully destroyed on its own. The relevant values in the object's Objects.con file are:
ObjectTemplate.CriticalDamage 12 ObjectTemplate.HpLostWhileCriticalDamage 1.5
The CriticalDamage value is the number of hitpoints at or below which the object starts to self-destruct. This is usually about 10-20% of the Maxhitpoints, though can vary on up to 50%. The HpLostWhileCriticalDamage value gives how many hitpoints are lost per second when in the critical zone.
Reaching critical damage will also disable any engines that are attached to the object.