In FH2 tanks, if the healthbar is near depleted, it stays that way until you bail the tank. If we bail at that condition, the health will continue to drop until it explodes. Usually, the explosion will kill the player/driver who exited from the top of the turret. Many bots died this way, and the kill message will say
killer <victim tank> victim
So that is how a PIAT soldier managed to kill a German tanker with Panther tank, at least from my experience.
You can't repair near-dying tanks if it has no drivers (other occupants won't affect this). If you tried to repair, while you keep tightening the nuts with your wrench, you may see a slight increase in the health bar, but it will quickly drop again to the previous level, making all you effort useless. If a driver is present inside it, the health bar won't drop and you can continue to repair it to full health.
edit: making it more readable