The answer to 'why is my supply line leaking water' is that a leaking supply line is a common plumbing issue that can be caused by a variety of factors, such as corrosion, improper installation, or physical damage. The symptoms of a leaking supply line include water dripping, pooling around the pipe, or even visible cracks or holes in the line.
To fix a leaking supply line, you may need to tighten the connections, replace washers or gaskets, or even replace the entire supply line. In some cases, the issue may be more complex and require the expertise of a professional plumber, especially if the leak is in a hard-to-reach area or if there is significant water damage.