This is caused by the top being down for long periods of time, rather than the top going up and down. You can wash the top and let it dry in the sunlight or use a hair dryer at low temp and dry the areas that are in question. I have owned many cars in my lifetime and 95% of them were Rag Tops, and this has appeared on all of them. The only car that I have owned that did not do this was my 1976 Caddy Eldorado, it had a headliner that was attached to the top in certain places. The headliner would cinch up when the top was retracted and stop the top from "bellowing" when down.