Hamstring tightness is caused from tight Psoas muscles.
The biomechanics of this are that your Psoas mucles are attached to your Ischial tuberosities ( your but bones). As the Psoas muscles compress and compact over our spine from very early on (as we start to sit): they start to produce Huge forces that can tilt the pelvis.
The tilt is Forward. So we develop an anteriorly rotated pelvis. This is like a guide wire in a tent being pulled more on one side than the other. the result is that we have tight Hamstrings and our Abdominal muscles appear to be bigger on the other side.
This causes the Hamstrings to be overstretched which then results in too much constant Tension being put through the Hamstrings.
So there is less flexibility and movement in them.
Posturepro can get you amazing hamstring flexibility