Assuming a submersible pump. Possibly a water leak in the drop pipe in the well. If you do not have a check valve at the pressure tank, shut off the water to the house and watch the pressure gauge over like 30 minutes. If the pressure falls it proves a leak. If not, you may have a leak on the house side of the shut off valve. So with no water use, watch the gauge again and if it falls, you have a leak on the house side which would include the irrigation.
If you have a check valve at the pressure tank, remove it, they are usually installed where there is no need for one or, if there is a leak as a temporary 'fix' which is wrong but it's done instead of pulling the pump and fixing the leak.