Flushing will accomplish nothing so I don’t understand the logic behind doing that.
The leak could be either an injector or fuel rail seal or possibly even an injector itself.
The injectors and rail can be sealed with new O-rings if that is found to be the problem.
As to why it’s leaking maybe a harsh additive, something in the gasoline, or even someone having been into the fuel rail before and inadvertently nicking an O-ring, or banging into an injector body a bit roughly could be behind this problem.
Texases is right; this is a fire hazard.