From my training in Neurokinetic Therapy (NKT) with David Weinstock, David refers to the iliacus as a "LCF", which stands for Lower Cluster F**k, because of its common involvement as a facilitated compensator for so many other muscles throughout the body. For example, it is very common for the iliacus to compensate for a psoas or gluteus maximus or abdominal oblique muscles that aren't working as effectively as they can be.
So why is the iliacus commonly facilitated?
I don't know exactly, but my best guess could be it's location. The iliacus is a deep muscle that inserts to the upper two-thirds of the iliac fossa and inserts onto the lesser trochanter of the femur; it also has connections to the anterior aspect of the sacrum.