Odd thing happening.

If I remember correctly, those "motion" detectors sense a change in temperature. Could the microwave be venting some heat into that cabinet?

What JKWilson said.....

I worked in the electronic security business for over 40 years. There are several types of motion detectors
a) PIR - Passive Infrared going into alarm when a heat source makes a sudden change in front of it. Microwave energy can set it off by saturation to the circuit board if close enough. Also vibration on the surface it is mounted on can set it off as well
b) Microwave - Goes into alarm when a solid object moves in front of it disturbing the received microwave energy. Another microwave source close by could set it off
c) Ultrasonic - Sound waves bounce off of objects and it looks for a shift in the received sound waves when a person walks in front. This is not used much anymore because all kinds of factors would set them off. Not sure if microwaves would interfere with these
* If your leaking that much microwave to set off a motion detector inside a nearby cabinet you need to look into this right away ... Try putting a piece of tin foil on the inside of the door and see if the microwave still sets if off, bet it doesn't .. that would indicate the microwave is leaking all over the place
*** Referaing to the piece of tin foil on the door, I MEANT THE WOODEN DOOR WHERE THE MOTON SENSOR IS, NOT THE MICROWAVE DOOR