Posture affects our necks negatively when there is anterior or posterior pelvic tilt because the spinal curves are altered and the head is carried in a forward position. The muscle at the front of the neck, the Sternocleidomastoideus (SCM) shortens and the shoulder girdle rounds and shifts forward, exaggerating the curvature of the upper back. In some people, the upper back remains relatively straight and the lower cervical curve reverses. Both of these neck positions cause pain in the upper back and neck.
Another factor in shortening of the SCM is crunch-style abdominal exercise: the neck is strengthened in an inappropriate anatomical position -which is going to persist when you stand upright.
Please refer to the post Mid-back Pain for more detail on the negative effect of abdominal isolation exercises. Abdominal exercise also strengthens and tightens the hip flexors, mainly the Psoas. A tight Psoas also causes a tight neck.
Pain and tightness on one side of the neck is sometimes related to the hip as well; a difference in leg-length or weakness of hip muscles causes the pelvis to tilt sideways and a correction will occur in the neck, in order to keep the head horizontal and preserve balance, resulting in strain on one side. One-sided neck pain and stiffness needs assessment and treatment by a Chiropractor.
Yogis should be aware that their practice can have a negative effect on their posture, postural problems such as the ones described are correctable with exercises prescribed by a movement specialist, and they can also be created by unbalanced posture sequencing, or incorrect movement habits, for example, habitually moving in anterior pelvic tilt, a very common habit that teachers need to correct when they see.
Sharkey, 2008, The Concise Book of Neuromuscular Techniques
De Franca, 1996, Pelvic Locomotor Dysfunction