Tight and weak hamstrings are two of the main causes of back and pelvic issues. Tight, retracted hamstrings can cause pelvic rotation and ultimately, loss of that sweet lumbar curve. In addition, weak hamstrings can cause the dreaded butt wink, or slide(shifting forward at the bottom of the squat), which also makes you vulnerable to a lumbar injury.
To combat this we need to make sure we are doing the right work to keep them in shape. Yoga is a great option, especially on an active recovery day to get a sweat going, get limber and find your crossfit Zen. So make sure you do your good mornings, hip extensions, GHD ham raises, inch worms, reverse hypers, and all of the other great hamstring mobility tools out there. Talk to your instructors, and check the main site for tips!