I would highly recommend everyone reading this article. It is covers only the chassis side of fires and the causes. It is a real eye opener. As somebody preciously psoted and lot of these fires are caused by no or incorrect maintenance especialy in the area or brakes and wheel bearings. The other big cause is electrical fires on the heavy cable from your high ampeargae alternator and battery cables. Lose or dirty connections cause resistance which increases current which generates heat.
This article is not guessing as to what causes the fires but is based on the results of investigations of fires.