It could be a few things. SLDL’s have a mobility effect on many muscle groups. They push the calves and hamstrings as far as flexibility limits goes, taking the two under load, in essence stretching them. Sometimes tight hamstrings are really just a byproduct of tight hip flexors or visa versa. This could be one explanation. Another is that at the top, if you are squeezing the glutes, you’re directly contracting the antagonists of the HF thus giving them a loosening effect. In the end, if it works for you, keep it.