Woodpelletsmoker, are you saying that your smoker is reverse flow due to the the flame flowing out of the air inlet once the fan stops? Actually that's flashback or backdraft, as the heated fuel seeks out air to burn in. Smoke has unburnt fuel in it and an (air) starved fire is a whole lot of unburnt fuel looking for air. In the right ratios the heat, fuel and air = explosion!
First let clear up reverse flow -
For most of us reverse flow is the smoke is made to follow a longer path through the smoker. Smoke flows from the firebox to the far end of the smoker and back exiting near the fire box. See below
Then let's look and flashback / backdraft -
This happens when hot fuel finds air, the fuel catches fire. The fire makes the air and fuel mix more as the heat makes the air more turbulent. This makes the flame hotter, etc it can reach a point where it explodes.
here is a flashback / backdraft you can try for yourself it won't blow up