I cleaned my hotpoint electric dryer vents every 2 - 3 uses. I never put too large a load in the dryer. My 7 year old dryer caught fire. When I opened the door to take out a few towels, I saw flames. The fire was contained inside the drum of the dryer. Any idea what could have caused this?

Yes, it was the lint that was caught in the venting to the outside. That must be cleaned out by hand every 6 months or so to ensure no lint build up resulting in a fire. More than likely your dryer is not repairable and you'll have to get a new one.

Just cleaning the lint trap out each time is fine and dandy but you must clean the venting ductwork to the outside and also the outside flap area very thoroughly. Clothes give off more lint than is trapped in the lint trap within the dryer itself. You'd be amazed at how much lint is still able to get through the mesh in the lint trap.