Birth trauma or intrauterine malposition is considered to be the cause of damage to the sternocleidomastoid muscle in the neck. Other alterations to the muscle tissue arise from repetitive microtrauma within the womb or a sudden change in the calcium concentration in the body which causes a prolonged period of muscle contraction.Any of these mechanisms can result in a shortening or excessive contraction of the sternocleidomastoid muscle, which curtails its range of motion in both rotation and lateral bending. The head typically is tilted in lateral bending toward the affected muscle and rotated toward the opposite side. In other words,  the head itself is tilted in the direction towards the shortened muscle with the chin tilted in the opposite direction.Congenital Torticollis is presented at 1–4 weeks of age, and a hard mass usually develops. It is normally diagnosed using ultrasonography and a colour histogram or clinically through evaluating the infants passive cervical range of motion.Congenital torticollis constitutes the majority of cases seen in clinical practice. The reported incidence of congenital torticollis is 0.3-2.0%. Sometimes a mass, such as a sternocleidomastoid tumor, is noted in the affected muscle. Torticollis does not resolve on its own and may result in plagiocephaly. Acquired torticollis
Noncongenital muscular torticollis may result from scarring or disease of cervical vertebrae, adenitis, tonsillitis, rheumatism, enlarged cervical glands, retropharyngeal abscess, or cerebellar tumors. It may be spasmodic (clonic) or permanent (tonic). The latter type may be due to Potts Disease (tuberculosis of the spine).
Known causes for head tilt in domestic animals include:

Encephalitozoon cuniculi (or E. cuniculi) infection in rabbits
parasitic infestation by the nematode (roundworm) Baylisascaris procyonis in rabbits
Inner ear infection
Hypothyroidism in dogs
Disease of the VIIIth cranial nerve the N. Vestibulocochlearis through trauma, infection, inflammation or neoplasia
Disease of the brain stem through either stroke, trauma or neoplasia
Damage to the vestibular organ due to toxicity, inflammation or impaired blood supply
Geriatric vestibular syndrome in dogs

Notes
References
External links
Head Tilt: Causes and Treatment