Pressure can change at any time. Municipal pumps change, problems with valves, pinched lines, cracked lines, etc.
Two weeks ago a lady had low pressure on 8 of 12 zones. No leaks, no obvious problems.
Eventually figured out that during our drought one section of the concrete walk to her front door had settled about 3 or 4 inches.
The main line for those zones ran under the walk and was being pinched off.
Another house, old system, tree had grown so much the root had lifted the pipe out of the ground. The pipe was bent up, over and down the other side of the root. Pipe pinched to about 1/2 size.