Diagnosis
An immediate circuit trip points to a direct short or a massive power draw. The usual suspects are a dead short in the high-voltage circuitry (diode, capacitor, or magnetron) or a failed component that's grounding out.
How to Fix It
Before you suspect the microwave, rule out your kitchen's electrical circuit. 1) Plug the microwave into a different, known-good 20-amp outlet on a different circuit (maybe using a heavy-duty extension cord temporarily for a test). If it works there, your kitchen outlet or wiring is the issue. 2) If it still trips the breaker on a different circuit, the problem is inside the microwave.
If That Didn't Work
If the microwave trips breakers on multiple circuits, it has an internal fault. This is not a DIY fix. The high-voltage components hold a dangerous charge even when unplugged. Diagnosing which part (magnetron, capacitor, diode, or transformer) has shorted requires specialized tools and knowledge. Time to call an appliance repair technician.
Heads Up
Never, ever try to open the microwave cabinet to investigate a tripping breaker yourself. The capacitor can store a lethal shock long after it's unplugged. This is one of the most dangerous repairs in home appliances.