I listed the most likely reasons that people chew their tongues. Unfortunately each possible cause or reason
justifies a substantial explanation as to why I listed it. These explanations are given in part elsewhere in this
web site ether under the Therapies, Q & A or the Habit Disorder sections.

1. An extension or variation of the thumb sucking or teething pattern, even if you have never sucked your
thumb. A non-nutritive oral behavior.  
See Article:  Oral Habit and Tension Patterns Correspond to Dental Growth, Speech, and Pain Patterns by Karen Alexander SLP, MAT

2. Insufficient lower or upper arch width or over-sized and/or enlargement of the tongue (macroglossia).

3.  Nerve damage, possibly due to trauma or tooth extraction.

4.  Malocclusion -  Open Lateral Bite  or Lateral Tongue Thrust

5. Hypoglossal nerve interference at the C1 vertebra.
See: Hypoglossal XII Cranial Nerve Connection

6.  A neurochemical imbalance of dopamine, norepinephrinein or serotonin in the brain.

7. A Sensitivity or reaction to certain food substances such as gluten, peanuts or sugar.

8. Side effect from a prescription medication.
See: Tardive Dyskinesia - bucco-linguo-masticatory syndrome", meaning “cheek-tongue-chewing” syndrome

9. A mimicked learned behavior.

10. Problems with the (TMJ) Temporal mandibular Joint
See Habits and Disorders: Temporal Mandibular Joint Dysfunction

11. As result of orthodontic treatment.   
See Therapies:
Craniodontics (relationship between the function of the teeth and the skull)

12. Impacted wisdom teeth

13. A Cranial dysfunction or misalignment:

The misalignment in the tension the fibrous membranes that support the skull. These fibrous membranes
are innervated with sensory nerves. Bruxism and possibly tongue chewing may be a result of the bodies
attempt to relieve this tension. Even if the patient has seemingly straight teeth distortions may be present
that could setup increased tension within these inter-cranial membranes.  

14. Neuroanatomical variant:

Possibly a variant in agonist/antagonist neuroanatomy altering nerve transmission to the tongue, causing
a large scale muscle fasiculation due to out of sync nerve trigger or firing impulse coordination.
OR…  A variant in the neurotransmitter speed (for numerous reasons, slow production, slow transmission,
reduce receptor absorption [due to another local anatomical variant] or reduce speed of neurotransmitter
release [due to a local anatomical variant]).

14. A Congenital Sensory Integration Dysfunction

16. Just a habit

17. Airway Restrictions

18. Any combination of the above.