Not a car expert, just a guy that learns how to do something when it needs to be done…
I don’t know how relevant the “why” is… you’ve got a leak, and the easiest first step is to inspect to see if you can figure out where the leak is originating from. If you can figure it out, then fix that. If not, the next easiest might be to just replace the coolant lines. If it’s still leaking after that, it could be either the radiator or a gasket.