Bio-mechanical issues, such as over-pronation and leg length discrepancies, which affect the way you walk can put excess stress on the hamstring tendons due to the imbalanced. Other causes that can put excess strain on the hamstring tendons are:

*strength differences in your hamstring, quadriceps or hip muscles

*lack of exercise and obesity

*lack of strengthen in your core muscles (i.e. abdominals and obliques)

*age-related weaknesses and/or break down of tissues

http://www.aidyourhamstring.com/hamstring-tendonitis/hamstring-tendonitis-causes.php