Largely cause by a lot of sitting, bike riding etc where you are bent at the waist for long periods of time.
The iliopsoas connects the bottom portion of spine and runs over the hip and attaches to the inner/upper thigh. Its really a huge muscle used for a lot of stabilization and bending at the waist/lifting the leg.
There are a lot of stretches, just youtube iliopsoas stretch. I personally like the thomas stretch where you have half you ass and legs hanging over the edge of a massage table or the like. Pull one knee up to chest and let other hang. If you have a friend who can apply resistance they can put a hand over the knee and you can do PNF work by lifting leg against their hand. You can also gently bounce the leg a bit.
Additionally foam roller and lacress ball work all around the legs, ass and glutes. Kelly Starrett also has some ideas for iliopsoas release techniques, some say they dont work but Ive had luck with them. I also find floss bands high in the crease of the thigh and up over ass cheek then moving through various ranges, leg swings front/back, then side/side, figure eights with foot, then perfect form air squats with some added emphasis at the top and over reaching to work that tight psoas.