Rodents chew on food-free bags (cloth, leather, PVC - whatever) for exactly the same reason they chew on electrical wire: To get the salt.
salt is a vital nutrient that all animals need for survival. For most non-humans who can't mine it or set up big seaside condensation ponds, it's hard to get. Historically, it was very valuable; that's why the word "salary" is allegedly derived from the Latin salarium - the money the Romans paid their soldiers so they could buy salt.
Livestock owners put out salt licks for cattle and horses; rangers sometimes do the same thing for deer. If you, a primate bicycle bag owner, have been handling your bicycle bags, there are traces of salt on them from your dried sweat. If you're a bag-chewing rodent, you've got to get your vital minerals wherever you can find them.
In the case of electrical wire, the charge of the wire crystallizes salt out of the water vapor in the air, which contains whatever trace minerals are in the local water supply. Uninsulated wire crystallizes better than insulated wire, but I assume that the shock of chewing on bare copper wire discourages rat/mouse/squirrel nibbling with pain and/or electrocution.
Peter Adler
who has lost multiple Ortlieb panniers in both Cordura and PVC to rats/mice in
Berkeley, CA/USA