One of the primary reasons for arcing in a microwave is the use of metal in the device. Aluminum foil, metal trimmed plates and metal utensils attract and concentrate microwaves like an antenna to create this electrical light show.
If there are no metal utensils or foil in the microwave, a damaged waveguide cover is a potential source of the arc. RepairClinic suggests removing this cover and testing the microwave to see if arcs continue to form. If this fixes the problem, the source recommends ordering a replacement, especially if the cover has breaks or burns.
Manufacturers use special paint to prevent arcing of the microwaves against the oven's sides. Chipping or pitting in the paint cause arcs to form. Special microwave paint is available for touch-ups to stop the arcing.
In microwave ovens with a rack, the weight of the rack sometimes pulls the supports from the side to expose metal. Appliance-Repair-It says removing the support, using touch-up paint to cover bare metal and installing a new support stops this problem.
In ovens with a turntable, the plastic lug that turns the glass plate is sometimes the source of the problem. Replacing the lug with a new one from the manufacturer usually stops arcs from this source.