When I moved out of my parents (who tumble dried their towels), I took a couple of towels with me, and found that they were rock hard after drying them indoors.
When I asked my mum about it she said that the reason they went hard was that they had been tumble dried before and now the natural drying causes the hardness.
I bought some new ones and have always dried them indoors and they are still as soft as ever, so the theory seems to work!
Also, line drying does not make towels as hard as indoor drying for some reason