Vehicles taking 80%-90% damage in a single hit

I have had it multiple times now where a tank or IFV will take 80%-90% damage to its health in a single hit front on. I have no idea what causes it, it seems random. Hitting a vehicle from the rear only does 50% so I'm not sure what is causing the 90% damage. It is not from hitting a mine or anything as you can be stationary and away from enemy infantry.