In yoga the neck is usually injured when falling out of Sirsasana or Salamba Sarvangasana but necks can become sensitive without any injury occurring, as neck problems can have origins in the hips, shoulders or overall posture. Isolation abdominal exercises which involve lifting the head and shoulders can also affect the neck profoundly by altering head posture.
One cause of neck pain is body alignment: the neck is sensitive to differences between the left and right sides of the hips, which causes lateral pelvic tilt, as well as postural problems related to anterior or posterior pelvic tilt because the head tends to be carried in front of, rather than above the rib cage. Dysfunctional Sacroiliac joints can also refer pain to the neck, as the neck compensates for twisting in the Sacrum.
Another major cause of neck problems are the shoulders: the Levator Scapulae attaches to the base of the skull and upper Trapezius attaches to the cervical vertebrae, as well as the base of the skull and any imbalance in the shoulders will be felt in the neck.
Neck muscles can also become chronically shortened if the neck is always hyperextended in asanas like Adho and Urdhva Mukha Svanasana. This is not likely to occur if asanas like Halasana, Salamba Sarvangasana, Pindasana, etc. are practised regularly but those who cannot practise these asanas because they have disc problems should be very careful not to hyperextend their necks during practice.
Having disc problems does not mean that people shouldn't stretch their necks at all, but need to follow advice resulting from professional assessment.
Stretching and strengthening the neck is on a separate page.