i've experienced this same stiffening overnight.
my theory is that it's because i use a case to store my headphones, and the process of winding the cable in causes a form of twisting with that portion of the cable above the Y splitter. it's not so apparent at either end of it, but i suspect that the need to loop that length down and back up (before winding the cable below the splitter into the central space) can cause a twist within the cover that overlaps the cables outwards, filling that space and making the cable go hard; if you hold the cable down before bringing it partially back up, it might last longer, but if the hold isn't done right you might essentially be bending and rolling one half over the other like a folding motion. yes, a hairdryer loosens temporarily, but it returns because it shrinks back around the cables. suspect it might be possible to heat and roll the cable to counter this, but as you can't see exactly what's going on and if you're correcting it accurately, i wouldn't like to say for certain.
i like the sound of the grado headphones, but i've paid out to replace one twist on an older pair (and resent it; i'm in the UK, so i don't get it free like the USA seems to) that essentially makes the price of them double, despite the ongoing design flaws that the company never owns up to or corrects. i will be looking to get a different pair if i need to replace this set.