A waterflow switch detects when water is actually moving in the sprinkler system.
When water flows, a paddle is pushed by the water, and that releases the adjustable air-damper. Because of changing water-pressure from the city, water will often push this paddle for a few seconds. Without the air-damper, this momentary movement would sound the alarms.
The air damper slowly releases air from inside the dashpot, and when enough air is released, them the switches themselves are activated.
In order to increase or decrease the delay caused by the air-damper (retarder), at the bottom of the dashpot is an air releasing adjustment.
The issue with the shortening delay is that part of the air-damper assembly is a diaphragm made out of rubber. Slowly, after years of service, this diaphragm will dry out and start cracking. The cracks will let the air out quicker, speeding up the damper movement, decreasing the delay time.
It doesn't help that the diaphragm is protected by the plastic enclosure of the air-damper, so the cracks won't be visible. Once the cracks start forming, however, the delay time starts to shorten. Adjustments to the air-damper are only a short-term fix. The only proper solution is to replace the whole waterflow switch assembly.
There is a good possibility that, once the cracks started forming, the rubber degraded rapidly, so, in 3 months-time, the delay can drop from :43 seconds to :05 seconds.
To prevent false alarms, the waterflow switch should be replaced.