Is there a general reason for tight hamstrings or is that to vague of an area to specifically pinpoint a sole reason for tight hamstrings? Could it be a result of sore hocks? Could it be a result of pain up higher ie: sacrum, hip, pelvis?
It could definitely be related to compensating for soreness in another area, but it could also be the result of conformation, poor riding, a slip, etc.
The lameness specialist that I went to after my jumper pulled his mentioned that it's more common in stockbreeds and warmbloods who have large muscle groups in the hindquarters.
I use extra caution now warming him up/cooling down, before I stick him back in his stall or throw him outside I'll massage some msm soothing gel in and do some slow stretching to try to keep the muscles from tightening up again.
I'm not convinced the gel actually does much, but you can feel it warming up when you massage it in, it was suggested by the vet, and it's fairly cheap, so I figure there's no harm.
It could also be due to the horse's hind end being out of alignment. My mare was rotated in her pelvis and then tight in her entire hind end. Once she was adjusted by the chiropractor all of her muscles loosened up.
My suggestion would be to back off the workload and if it doesn't go away, or gets worse, call a vet for some diagnostics.