As Eric alluded to, once you get a tear, you are more likely to get another. This is due to the scar tissue forming, which doesn’t have the same contractile properties of normal muscle fibre. The research on static stretching seems to show that it has little effect on dynamic flexibility. Ive also seen a paper on hamstring injuries and static stretching, which said that static stretching did not prevent hamstring injuries. Having said that, I do think there is value in stretching your hamstrings (and all major muscle groups) as Matt states.
A lot of hamstring issues can come from the lumbar spine (which can be too stiff, or too mobile) or from the pelvis (which can be rotated forward or clockwise/anticlockwise). You might also have a leg length difference, which could be causing these issues. Im sure you have been getting physio to work on these injuries – what do they have to say?
I know you do a lot of specific hamstring strengthening exercises, which should protect you too. Do you know which hamstring muscle you injure (i.e semi-mem, semi-ten, or bicep fem) and where abouts in the hamstring it happens (top, middle, bottom)? Do most of your tears happen at maxV, or acceleration? Or round the bend?
Its possible your motor firing patterns are off? Do you do glut strengthening exercises too? How about rotational torque control through the torso (basically side planks, wood chops etc.)?
Thats basically a decent overview of rehab protocols and injury mechanisms – its all I can think of off the top of my head. The next step would be to look in detail at recovery, sleep, fatigue, nutrition, all other factors that could be causing this.