The cooking rack gives you extra space when cooking in more that one container at the same time. The rack should only be left inside the microwave when cooking food on both levels, otherwise it should not be left inside the microwave or this could possibly happen. Once the rack holder becomes burnt and carbonized it will continue to spark and burn. The rack holder will need to removed and replaced. Part number for the rack holders is 4393849.
I recommend calling a service technician to remove and replace the rack holders and to check over the microwave for any other possible damages.