Hydrocarbons and additives which forms part of today’s fuels evaporate at different temperatures. Small amounts of gasoline are left behind on the tip of the injectors each time the engine is shut down. Volatile compounds in the fuel evaporate and while others stay behind on the injector tip and eventually form solids. These solids accumulates and eventually start effecting the spray pattern and fuel distribution which in turn can have detrimental affects on the overall well being of the engine.
Higher operating temperatures of turbo and supercharged engines are very tough on injectors, especially due to higher intake temps and sometimes reversion. The same is also true for high performance engines with large duration cams.
Many high performance air filters does not clean the air sufficiently and contributes to the fouling of injectors. Dirty fuel filters and failure to regularly replace them is also a contributing factor
Moisture in the fuel tank that could eventually lead to rust being deposited below the injector filter and can cause catastrophic failure of not only the injector but in some instances the engine
Engines with worn rings and valve guides greatly contribute to the clogging of fuel injectors.