There is no reason for it. It is called 'age'. I suffer the same illness.
Yes, there are plenty of things that creak and need lots of oiling at my age but having just one hamstring be tight and not the one that I normally have problems with suggests I did something out of the ordinary to that leg.
I don't know what I did to my calf muscles but they're both pretty crap.
Having dodgy calf muscles is definitely old age.
Not yet but heading that way.