Simply put, you have created a ground loop by letting the motor currents flow along the "ground trace" on the PCB that contains the PIC. I never-ever allow current from a high current (or inductive) device to flow along a conductor that connects to an analog ground, like an ADC or PIC. I always carry the high-current ground separately all the way to the negative pole of the supply (or battery).
Read up on "single-point-ground", "star ground", or "ground loop".