I've got an Amana built-in microwave that has started on its own intermittently ever since I got it 14 years ago this week. It only happens when I close the door, not spontaneously. I've never worried about it, but maybe I should.

And Pardes, I'm so glad you didn't have a serious fire this time, but I'm having a hard time getting my head around how you could be living in a house without working smoke detectors after losing everything to a fire 6 years ago!