Couple of issues come to mind. 1. You are probably quad dominant and your hamstrings are probably weak. Time to lay off quad based exercises and put more focus into the posterior kinetic chain. Doing eccentric based movements for the hamstrings will help to prevent injuries. Look up nordic eccentrics and do more glute ham raises. Next, strengthen the hamstring at the hip with good mornings and RDL's.
2. The hamstring fibers are not arranged in a penate fashion, instead they are fusiform. There's no central portion of the belly where the fibers fan out, instead they run in parallel. This means that they contract very quickly, but their maximal force output is low. Compare this to the quadriceps, where the fibers run pennate to each other. Now, the glutes are also a pennate muscle, and really should be a primary hip extensor during much of sprinting, especially acceleration and change in direction (such as during rugby). Your issue might be that due to weak underactive glutes your hamstrings are recruited to work above and beyond their capabilities. This leads to weakening where the tendon inserts into the ishium and eventually tears...which, unfortunately, are often reoccuring with hamstring injuries.
So, the moral of #2 is strengthen your glutes.