Way back when I used to own a VW Squareback. The thing had a "leaky" injector. Basically it stayed open all the time. If the ingition was turned on but the engine wasn't started it would try emptying the gas tank into the engine. The way I tracked down which one it was was by squeezing the fuel lines by each injector and listening for the sound of fluid dribbling into the manifold. Sort of like milking a cow. (something about being from Wisconsin I suppose ). I pulled the one where I heard the noise, replaced it and the all was well in the world. That car also was equipped with a "cold weather" start valve. Basically and extra injector that would fire until the engine was up to temp. I think Subaru uses a different method for the cold days though (injector pulse duration?).

You may have had one injector stuck open that let the fuel in the lines drain into one of the cylinders. Cleaning might help unstick it.

Good luck, it's the kind of thing that can drive you nuts.