If you have any turn input, i.e. your rudder pedals or twisty stick are not perfectly centered, the automatic transmission will shift into forward. Hold the S key down to stay in reverse while turning backwards. When you stop, if your pedals have any spiking at all the only way to prevent the tank galloping off by itself is to turn off the engine. That means, of course, you have the delay in turning it back on if you have to move fast again.
I've got an even worse problem: while driving at full speed in a straight line the tank will sometimes make a hard right turn by itself. I don't know why; I've watched the twisty stick reading in the Control panel and I've never caught it spiking. I guess my tank is possessed.