Because there is a leak in the pipe of the water main.
You may have a leak in your tray where the water dripped out before it froze completely.
yes and no, if the water pump seals are bad antifreeze will leak from the weep hole on the water pump, but it is possible for the pump to go bad and not leak at all.
A possible leak
It would be possible to cause an oil leak while replacing the water pump.
You need to identify the leak before you can fix it.
If you have been running your air conditioner, it's possible that your drain hose is clogged, and the condensation is dripping in. It is also possible that you have a leak in the heater core or a windshield leak.
yes, leaking coolant is what happens before the pump totally dies, then you have overheating and possible engine damage soon after the leaking.
Possible freeze plug leak at rear of engine block.
You can try putting some "Stop Leak" in your radiator, but the only proper way is to remove the water pump and replace the gasket and the Pump too if it's leaking.
The water could be coming from a coolant leak higher than the plugs on the engine. Look around the head for signs of any leak as it is possible that there is a head gasket leak.
Radiator or hoses. Possible water pump