If the power supply to the load is not de-coupled properly, it is possible that it is causing a brown out on the supply to the micro, or perhaps the load is putting down a transient that's pulling the gnd(Vss) below zero, most micros would become unstable with either condition.
As everyone has pointed out, fit protection diodes everywhere they are needed, ensure that the power supply you have can actually handle the load properly, make sure the load you are supplying has sufficient decoupling and feed your micro's Vdd with a small coil and a seperate decoupling capacitor.