Modern work and technology has resulted in very poor posture for many of us. Sitting at a computer day and constantly looking down at our mobile devices cause postural muscles to thin and weaken.
Most dominant is the head being too far forward of the body. The constant load on the shoulders holding the weight of the head in this position causes the muscles to cease up. Muscle knots and trigger points form.
This can go on for years. The knots and pain on the top of the shoulders and between the shoulder blades can become a permanent feature of you.