When I move my platoon into a defensive position and sit for awhile, my tank will start moving forward by itself without a command. What gives? Is it a glitch or is there some command to give to keep it still?
Right now I shut the engine down, but that's very hazardous to my health in a combat situation.

Only 2 things can make the player tank move.
#1 a keyboard input to accelerate.
#2 a joystick movement.
most likely #2. PE's input code is very old. Try setting you sticks dead zone larger or unplug it all togther.