The answer to 'why is my supply line leaking water' is that a leaking supply line is usually caused by a worn or damaged washer, corrosion, or improper installation. Common symptoms include water dripping from the connections, water stains around the supply line, or a sudden spike in your water bill. To fix a leaking supply line, you'll need to turn off the water supply, disassemble the connection, inspect and replace any damaged parts, and properly reassemble the line.
If the leak is minor, you may be able to fix it yourself by replacing the washer or tightening the connections. However, if the leak is significant or you're not comfortable working on the plumbing, it's best to call a professional plumber. They can properly diagnose the issue and make the necessary repairs to prevent further water damage.