The dryer then overheated because it had to work extra hard to dry the laundry. A small piece of lint inside the dryer vent line ignited and traveled upwards; which caused the lint filled vent line to ignite.
Fortunately the fire department was able to extinguish the fire before further damage to the building could be done. This fire could have been a horrible disaster if the fire had caught the unit on fire and then traveled to the other 7 units in the building. This could have all been avoided simply by getting the dryer vent exhaust line cleaned.
Dryer Vent Wizard of Middle Tennessee