The irrigation system has been winterized last autumn. A few weeks back, upon refilling it with water I have noticed that most of the valves were less or more leaky. Surely enough I was able to turn the diaphragm screw and they stopped leaking water through the sprinklers / emitters.

What may have happened ? This is my second winter I am going with this system. This symptom has not manifested itself after the previous winter.

Thanks