This is how a catastrophic dryer vent fire occurs. First and for most lets clear a common misconception. Almost all Dryer vent companies in Central Florida will advertise that cleaning your Dryer vent will prevent a Dryer vent fire. This statement is a half-truth.
The clothes dryer heating element is actually where a dryer vent fire begins its catastrophic destruction and ignition. Yes a clogged Dryer vent causes a fire hazard but maybe not how you may think. When the dryer vent is clogged and is not able to exhaust heat air and lint freely to the outside of your home. The issue if your dryer vent system is clogged the dryer is still pushing that heat air and lint somewhere. This is when “Backpressure” happens. “Backpressure” of lint in the dryer vent world is the real cause of the fire hazard. Lint that cannot exhaust out doors will now push back into you dryer cabinet passing though leaky seals where you cannot see. It is impossible to know the condition of your dryer internals without disassembling your dryer and doing a quick inspection for flammable deposits near on or in areas that get extremely hot. Annual inspection of your Dryer heating element is highly recommended.
Inside your dryer is a heating element that reaches temperatures over 1000 degrees Fahrenheit. I hope your starting to get the picture. The lint in or on the heating element ignites on fire first. Next the fire uses the lint on the floor of the dryer as fuel and eventually works its way into the Dryer vent exhaust system. If the Dryer vent is full of flammable lint then the fire continues its path of destruction.