Lawnmower fuel system question.

I have a John Deere riding mower that is having a fuel problem. The mower will run on starting fluid, so I know it is a fuel problem.

I pulled the fuel line off between the fuel pump and carb to test the fuel pump, which is working fine.

I plug the fuel line back up between the fuel pump and the carburetor and it starts fine now.

Any idea what the cause of this was? All I did was basically unplug the fuel line and plug it back in?