Tight hip flexors occur for a variety of reasons. Those who run frequently or engage in other activities that put strain on the hip flexors are likely to experience hip flexor tightness at one time or another. A blow to the hip or poor conditioning can also be causes of tight hip flexors. These causes can usually be attributed to tiny tears that occur to our hip flexors through rigorous activity.
The problem is that these muscles aren't designed to be prime movers—they're designed to support the action of the glutes. Inability of activating the glutes can result in low back pain (low back muscles compensating), hamstring strains (overacting hamstrings), hip pain (resulting from hamstring-dominant hip extension) and knee pain (poor glute medius strength).