Because there is a leak in the pipe of the water main.
you may have a septic system leak or a water main leak
water leak
you find out were the water is coming from after you fill it up with water.
Posibly there is a water main leak nearby mixing in with the ground water.
assuming that it is a waterline leak, i would suggest the alternate route for simplicity sake.
One can find a cheap water leak detector from their nearest hardware store. Online retails like ProfessionalEquipment, HomeDepot, Amazon and eBay also provide cheap water leak detectors.
By applying pressurised water at one end of the pipes. The water leak can be found out and it can be sealed by using adhesives.
Water leaks can be tricky. If water is visible on a wall then it could be necessary to cut into the wall to find a leak. Listening in a very quiet house may allow you to hear a spraying leak. If a leak is suspected because of a high water bill, look at the water meter. The small dial moves when water runs. Shut off cut offs under the sinks and toilets and the water should stop running. Turn them on one at a time and watch until the meter tons again. There is your leak.
I suggest taking it to the auto trim shop as it is hard to find the leak and hard to stop it and they do it all of the time. Roof Leaks identification and material selection for repairing roof leaks is a main concern.
You can do it two ways: Use soapy water while it is inflated and find the leak or immerse it in water as you squeeze it to exhibit the bubbles from the air leaking out.
Water can not leak from the rear main seal, only oil. Most likely there is a leaking freeze plug near the rear main seal dripping water onto it.