maybe, but arson forensics is a pretty advanced field. there's a lot of motivation for arson, and the police and insurance companies have a lot of motivation to identify it. i'm not saying that this was arson, just that we have a lot of skill in this area.
there have been cases of clothes left in a dryer starting a fire. there's enough heat to smolder a piece of lint, at which point it's all over. if there were a splinter in just the right (or wrong) place, a doorknob like this could pretty easily spark it.