Aging. Number 1 enemy of any moving part. The closing valve pins controlling the petrol flow wear out overtime as well as the spring that pushes it back in close position every time a pulse signal to the solenoid pulls it up which allows the fuel to spray out into the cylinder.
As the years pass by, fuel injectors tend to run rich or leak (due to valve pin damage, or worn spring, or in worst case scenario, both!) . The ones that I took out from my car looked old as hell and I wonder if the previous owner ever changed/cleaned them in the life of this car except for the brown one.