Motors on autonomous driven vehicle free spin in opposite direction they are turning while powered
We can give this vehicle a drive command, which it follows. When we
  apply force in the opposite direction, the unit will roll in that
  direction. As soon as we let go of the AGV it continues to drive
  forward again.


Did you remember to remove the "Drive" command? You don't say that, only that you pushed it in the opposite direction and when you ceased, it resumed its previous motion. If the Drive command is still valid, that sounds as though it is doing exactly what you told it to do.