Generally dryer fires start from one of the following places:
Lint Built-Up and Clogged Ducts:
The most common cause of dryer fires is the result of lint build-up in the dryer and exhaust duct. When dryers are not cleaned on a regular basis the lint build-up restricts airflow, and when the airflow is restricted, clothes take longer to dry. Clogs and restrictions prevent the air from circulating and the dryer can't sense the normal operating temperature and so it continues to rise. This is the first indication that there is a problem.
Dryer Ventilation:
If heavier clothes, towels, and bedding are taking a long time to dry or clothes feel hotter than usual at the end of the cycle, a clogged dryer vent exhaust is likely the problem.
Overloading the dryer with too much clothing or bedding materials can also cause dryer fires. The overloaded dryer cant move air through the drum and causes it to overheat and start to burn.
Faulty installations also can cause serious fire hazards. If the dryer vent exhausts into areas other than the exterior of the home, mold buildup and decreased air quality can occur.