Diagnosis
A ground fault within the microwave. Moisture or a wiring fault is causing current leakage to ground.
How to Fix It
Unplug the microwave. Inspect the power cord for damage. Check the area around the door and control panel for signs of liquid ingress. Clean any moisture from the cavity and around the door seal thoroughly. Let it dry for 24 hours before plugging into a non-GFCI outlet for testing.
If That Didn't Work
The high-voltage capacitor or the magnetron may be developing an internal short to ground. This is a serious fault requiring component-level diagnosis and replacement. The microwave must be disconnected from power and discharged before inspection.
Heads Up
Do not bypass the GFCI outlet or use an extension cord. A persistent ground fault is a shock and fire hazard. Discontinue use until repaired.