We know that the primary function of the hamstring during ground based locomotion is to extend the hip. Placing substantial load on it as a flexor of the knee, in my view, places a non-complimentary stress on it that when combined with the stress of high speed locomotion increases the likelihood of pulls.
This is the theory that makes sense in my system and I have a fantastic track record of hamstring health with my players during the times of year in which I have autonomous control over the entire training load.
It is not my intent to discourage others from performing GHR and hamstrig curls. To each his own.