Is there any chance that you have an error in the RS485 hookup wherein the drivers at the PC adapter and the drivers on your board are both on at the same time but one wants to pull high and the other low??

This situation can lead to large amounts of extra load on the puwer bussing of your board and on the power supply. It could be that such extra loading is making the power supply to your microcontroller chip and supervisor be marginal where they now become very sensative to any extra noise when the motor is activated

Michael Karas