Could it have something to do with the torque converter locking in auto stick mode? I know you can get some engine braking effect if you downshift with the auto stick. I can get the car to lurch forward and backward at slow speeds if I am not smooth with the throttle.

Maybe it's not the torque converter, but some mechanism seems to link the engine to the tranny the way a straight drive does. My Dodge diesel automatic does a similar thing in the tow/haul mode.