Assuming your hinges are tight, your bottom door switch is probably defective, or its mount is loose, misaligned, or broken.
This is usually caused by opening the door while it's cooking (instead of hitting "stop" first) or slamming the door.
I have the service manual at my site
here. See my help file on
door switches here.
More questions? Contact me directly
here.*
* always include the
full model number and FCC ID number from the label on / in your microwave (or a clear photo of the tag)