Dryer fires are the result of lint building up in places other than just your lint trap. While your lint trap is designed to catch most if not all the lint, it is not a 100% guarantee. Oftentimes, the lint gets lodged in other areas, such as the interior dryer duct, the dryer vent, or even the dryer itself.
Once the lint accumulated continues to increase, it starts to block the airflow within your vent and raise the temperature. If it becomes too hot, your dryer may end up catching on fire. This unfortunate turn of events affects thousands of homes per year but is highly preventable with regular dryer vent cleaning.