A lot of hamstring injuries come from muscular imbalances which we all have. The focus on alignment in Iyengar method of Yoga helps one to understand the imbalances and work towards correcting them to promote optimal range of motion for healthy joints.
Julie Gudmestead is a certified Iyengar Yoga Teacher who integrates the healing benefits of yoga with her medical training as a Physical Therapist. I highly recommend reading Julie's article Body of Knowledge - Understanding your Hamstrings.