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.
There’s something commonly called force-couple relationship. I don’t think I can explain it well, but basically if two muscles are working in opposition at an area (such as your hip), they have to have a balance - if one is too weak/ too loose or too tight/too strong, the other has to try and “take up the slack” in the system to create the appropriate stability (or mobility). Sometimes hip flexors are tight because the hamstrings and glutes are weak or because they’re too stretched, or a combination of both. What you’re describing makes sense to me. Stretching the hip flexors and strengthening the glutes and hamstrings often is a good combination, especially for folks that spend a lot of time seated.