I would guess that those last few grinding miles did it. When i 'hit the wall' , if that's what its called and that's what happens, i suddenly get insanely stiff, it feels like am not running anymore, no joints no tendons no bounce, just force pushing my body with leg muscles. Doing a few miles like that with the added muscle fatigue of another 20+ PR pace miles on your back, would make perfect sense to cause a total quads annihilation to me.