Fires occur very often when the clothes dryer is started.

Lint Tray is a common culprit - but also keep in mind that the AI Doesn't like to do it on it's own (I've run into situations where AI-Controlled Sims forget to clean the lint tray after drying and start a fire - which I believe was related to my earlier thread on "soft-crashing deleting characters" although the circumstances in that case included Player-controlled Travel in addition to the washer/dryer fire by AI Behavior.