Due to the plastic around the arches it holds water.
If there is any way the metal can breathe it will cause oxidation. Then the rust will start.
On the plastic trims most have holes for the water to run free, even if no trim you should always keep arches clear and clean.
If you have rust spots appearing treat as soon as possible. Sand down any visible rust - apply a rust inhibitor like
http://www.rustbulletuk.comAllow to dry, apply primer then colour code.
hope this helps
Dave