I agree with all of this although I expect dynamic stretching is still pretty relevant (I never said static) as well as warm-up activities with stretching elements.
Overtraining or functional imbalances seem most likely to me. Undertraining, unless somehow hamstring specific (which then is an imbalance) would probably show more general fatigue in game right? This could then lead to the injury - but most have been early game so unlikely, I think.
Can you tell me why a dynamic action like jumping, which doesn't really translate very specifically to playing, couldn't be a factor in functional imbalance or overtraining of specific muscles?
The jumping in a few recent training albums was just something I noticed and now, looking at all the hamstring injuries, I thought it seemed within the realm of possibility that if significantly more jumping was taking part at training that explosive action could be leading to an imbalance that is not translating well to play. It was just a random idea but I'd be interested to hear why you would comprehensively rule it out.