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][1] linked by [Pierre Chevallier][2] 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


  [1]: https://www.youtube.com/watch?v=0HDIxzSMp-0
  [2]: https://aviation.stackexchange.com/questions/44198/can-anyone-explain-why-and-how-flames-shot-out-of-an-f-a-18-as-it-crashed#comment113928_44198