I was thinking about this problem, and the reason came to me while re-watching the movie "Backdraft." It sounds like the same principal as a backdraft fire. Essentially, you have just enough oxygen in a space to barely support combustion. Our smokers fit that bill...small hole in the bottom (cool air in), large hole in the top (hot air out). The volume of air coming in is less than what is going out, but more dense air (cooler), so that probably equals out. Now, we apply heat to the wood, and the small amount of oxygen just allows a smoulder. Open flame eats up a lot of O2, and would (should) quickly consume the O2 inside the cooker, causing the flame to extinguish.
Here's the backdraft concept: What happens when you open the door to peak/spritz/check/etc.? A big rush of O2! If the wood is a little too dry, that flood of oxygen will cause the wood to ignite (it now has fuel). No door opening, no fuel. It would be interesting to know how many have experienced this scenario, and if it was after opening the door (even a crack).
I believe properly seasoned wood is the key, because I usually open the door during a smoke to spritz. I've never had chunks catch fire. Chips would probably be more susceptible to combustion.
For those of you that have seen the movie: The difference between our smokers and a real backdraft fire? Probably not going to blow you through your car window when you open the door! ;D
I'm not a fireman, but I did stay at a Holiday Inn Express last night! lol