Here is the way I see it, which could be way off, but hey, it's free . . .
With all the field ack shooting at him, he was probably getting pegged big time even though your film doesn't show it. Right at an inopportune moment, his left track is taken out by the auto acks. As the vehicle leans, your rocket hits. The trajectory appears to me to be through the track on the right side, and then the hit sprite appears on the rear of the tank. To me this indicates the hit took out his track, and still had enough power to partially penetrate the rear hull and engine. Because you did damage to the rear hull and engine, you see the hit sprite. But the damage was't enough to be fatal because most of the damage was dealt to the tread.
A moment before the tank rights itself, both tracks are "lifted", indicating he was double-tracked. As the tank rights itself, both come down, but again I think that is a glitch in animation rather than an indication he was suddenly "fixed."