Springs wear out, you know your car uses many springs and over time and repeated use through compression and release cycles, they break or loose their ability to provide the required resistance or force.
Same for any spring, some springs are made better than others and some are not and in airsoft, don't expect the best quality. Remember where they are made and the type of steel that is used isn't the same as home grown steel.
In Airguns you expect something that lasts the lifetime of the rifle but even then, they can fail, my sons HW57, a springer .177 had a failure of the spring because the rifle dieseled and caused the spring to become over stressed and it turned a sub 12 fpe in to a sub 8 fpe rifle.
Springs do fatigue and lose their elasticity / tension strength and if you have an ETU that applies precocking to the piston, your spring is under pressure waiting for that release to relax. I had this problem in my RIF where I had the precocking set but didn't let the tension go and what happened was the ratchet and spring went, I had to have a new spring and piston.