I think I heard that when lines are stored wet, mildew and mold grow within the line and they get stiff. Other than that, I liked the viagra explanaton. LOL
Your nylon lines are exposed to ultraviolet rays and the sun will make them stiff along with putting a load on the lines. All this will tighten the yarns that compose the line. Just like life the older you get the stiffer "certain" things get. Have a good day.