Dave-

Hamstring tears are usually the result of poor glute activation and overactive and/tight hamstrings. My guess is that you have kyphotic posture although you would want to find out for sure. Look in a mirror from the side and see if your butt tucks under your pelvis. Then, see if your hips are tilted back a little bit. That would put your hamstrings in a constant shortened position. Then, when you go to run, they are not capable of handling the load and tear.

I would suggest figuring out your posture distortions with the help of a physio and then work on specific exercises and stretches to balance those out. That should help a great deal!