I believe this was covered extensively in a previous thread. Remembering my firefighter training and physics class, and for refresher, gasoline has a flash point of -45F. Flashpoint is defined as a temperature above which the liquid produces vapors which can ignite or explode. This may happen without a spark or flame. Ignition temperature also applies to a hot surface (exhaust pipe) that will cause vapors to ignite.
Now, bearing in mine that gasoline has a vapor density heavier than air, it will sink and spread out. Vapors are what ignite when gasoline is poured out, such as putting gas on a pile of wood (dumb to start with). That is what causes the "whoosh" sound, the flame front traveling through the vapors. In training, we would drop a match in a container of gasoline and the match would go out without causing a fire.
Gasoline heated to the boiling point of between 100F-400F the vapors can be somewhat lighter. If the fuel tank is heated (we know what that is), it becomes pressurized. When the cap is removed, the pressurized vapors are forced out and up. Ignited, this is the "jet" effect you see.
Since there was an issue with brake cylinders I'll mention that most brake fluids have a flashpoint of 275F. So you can see that brake fluid on a hot exhaust pipe can ignite.
