Jesper, Denmark
Temperature swings and relative humidity. As the fabric insulates somewhat, tools stay cold and water condenses on cold tools. The fabric creates a miniature cold environment and keeps the moisture in.

The principle says that an RH <50% is safe, so if your shop environment meets that, all you need is to keep the roll somewhere with good aircirculation. If you cannot guarantee that environment, locating the tool roll on something that generates just a little heat will solve the problem.

If you have significant temperaure fluctuations daily (>10-15°F) and RH>70%, nothing except a protective coating will stop your tools from rusting.