The urethane coatings on the fabric eventually just break down. Same holds true of an ordinary camping tent floor (which is generally the only part of a big tent so coated).
As already noted, moisture, DEET, tree sap, any number of chemicals can cause a problem. I haven't used my Harrison tents in a long time, but made certain they were very dry and clean when put away into storage. When I recently inspected them they were fine. A much nicer tent and more functional than the Calumet. The "floor plan" is the key.