Faucet leak when irrigation turns on???weird

[Kgonzalez]

Went to a call on sat morning for a possible slab leak and found a leak in one the toilets was running water fixed the ballcock and meter in front stoped moving when problem was addressed . When I checked irrigation line and turned on ...the only bathroom in first floor lav sink faucet started leaking ?is a wide spread faucetand when I turned off irrigation it stops leaking ???i checkedpsi it's at 80 all good.. weird probably bad cartridge ?

 

[Mr_David]

That is odd.
Ceramic disc cartridge?
Those still amaze me how 2 hard surfaces can slide across each other and stop water from passing through them.
Maybe one pair is a little loose and when pressure drops, it's just enough to allow leakage.
Just a thought