Ben's explanation is dead-on. I've never experienced this in my #1, but encountered it during the first test run on the electronic #2! Scared the crap out of me. I was standing near the unit, looking at the temp display, and smoke started shooting out from everywhere! All around the door, and both holes, for 3-4 seconds! I was using the SI hickory dowels (new ones, so they were fresh).
Since then, it's been much less, and I've been "stepping" the heat up. I initially set it to 130, let it get to temp and cycle into maintenance mode. After about a 5 minute rest, I step it to 170, repeat the process, then up to 225. A more gradual warm up has stopped the combustion problem. I use Fruitawood cherry and SI or Fruitawood hickory mostly.
I believe this phenomena only happens in the 2s and 3s, due to the much larger elements. When it's at full power, it increases the chance of igniting the wood. Sounds like there are several ways to solve this, as we see here!