As the muscles get tight, the blood flow to the muscles becomes slightly restricted and there is less oxygen and nutrients getting to the muscles to keep them healthy. A lack of energy and nutrients to a tight muscle weakens that muscle and the muscle tightens up further. With tight muscles, you will be more prone to injury during daily routines and sporting activities.
Over-training can cause tight muscles. Too much training can cause muscles to strain and often results in small micro tears to the muscle fibres. Strained muscles will tighten to protect themselves from injury thereby reducing the blood supply to that muscle. This therefore restricts the amount of oxygen and nutrients getting into the muscles causing the muscles to work less efficiently.
Poor and/or static posture can also lead to tight muscles. If you have poor posture and muscles are held in a static position for long periods of time then this puts considerable pressure on the surrounding muscles, increasing the work that they have to do to overcompensate for the poor posture. These muscles are working twice as hard and almost become over-trained. Static posture throughout the day followed by post-work exercise means that the tight muscles are more prone to injury.