This happens very often with a large portion of the population.
Im starting to wonder, if the excessive tightness is caused due to a lack of strength compared to other muscles. If it cannot combat the problem with natural strength, it spasms and tightens in an attempt to combat other stronger forces.
That said, Flexibility and tone are looked at and corrected often and still tightness happens.
Which leads to a Q. - What kind of strength is missing, absolute or endurance?
An example could be the Rotor cuff of the shoulders. One is best working on the range of motion in a Strength Endurance phase than a Absolute strength phase. Same could be said for the hips flexors?