Door latch opened - what happened?

The door of the Harman XXV opened. This has got to be serious. The stove was burning fine but when I got up this morning the door was open, the motor was still running and stopped a couple of minutes later. The hopper was half full and the carbon monoxide/smoke alarm did not go off. I scraped off the burnpot as usual a few days ago and the stove was having no problems.

I started the stove again. Seems OK. Has anyone else had this problem?

It is just barely possible that I didn't close the door properly. That's all I can think of, but why did the stove run OK for a few hours? The latch doesn't seem loose at all. I hope it was just some minor mistake I made or else I've got a real hazard. Thing is, I'm mostly a careful guy about stuff around the house -- do an oil change according the car manufacturer's schedule, have the stove cleaned professionally once a year, etc.

Thanks for the comments. If anyone else reads this post and has heard of a similar situation, please let me know.