Can anyone explain why and how flames shot out of an F/A-18 as it crashed?
Fuel has to reach the engine to make it work, so there is a path (pipes) available for the fuel to get from the tanks into the engines.

Looking (*) at the video linked by Pierre Chevallier it looks like some fuel got pressed out of the tanks, into the engine(s) and out the engine nozzle, and in the process it got ignited.

I cannot say why the fuel got only in one engine, I can guess that the valves on the fuel lines were in different positions.

(*) I suggest using the option to visualize at x0.25 speed, starting at the 0:30 mark