What happened on my microwave was that after a while, the switch alignment was a little off between the three door switches, which caused them to actuate at slightly different times when I opened the door. This was totally fine if the microwave wasn’t running. But if I opened the door while the microwave was running, what was supposed to happen was all the switches would get tripped simultaneously causing the microwave to stop; what would actually happen is they would trip at slightly different times, which would cause an overload in one of the switches, cause the switch to fuse together like you’re experiencing, and cause the breaker to trip. I’m not an electrician and don’t know how to technically explain it, but all I know is as long as I press the stop button before opening the door, the switches stay fine and the breaker doesn’t get tripped. I hope this is helpful.