What causes a person to do this?

[IIIII]

I heard it might be something called 'Coke Jaw.'

But also it can be caused by anti-psychotics.

Tardive dyskinesia (TD) is usually a late-developing, well-known neuromuscular adverse effect associated with the long-term use of first-generation (typical or conventional) anti-psychotic agents, such as chlorpromazine (Thorazine, Glaxo-SmithKline), haloperidol (Haldol, Ortho-McNeil), and fluphenazine (Prolixin, Apothecon). After months to years of therapy with dopamine-2 (D2) receptor–blockers, patients often experience involuntary choreiform, athetotic, or ballismic dyskinetic movements. These movements most commonly involve the mouth, tongue, facial muscles, and upper extremities. Axial dyskinesias may also occur.1

http://www.democraticunderground.com/101743554#post5

But its weird looking no matter:

George W Bush Facial Expression Supercut - YouTube

George W Bush Facial Expression Supercut - YouTube
Video: click to display

[southernsun]

Looks like you answered your own question thar.

[Struwwelpeter]

Not everything has a logical explanation you flea-bitten mongrel

[DiStOrTiOn]

restless jaw syndrome

[STEROS]

tmj?

[constantinople]

I like to move parts of my body rythimcally when I'm thinking too. NEver taken any anti-psychotics.

[007GoldenShower]

2:00

Silverchair - Spawn Again LIVE - YouTube

Silverchair - Spawn Again LIVE - YouTube
Video: click to display

This guy was heavily medicated at the time (literally)

[Cunto McFadyen]

Originally Posted by constantinople

I like to move parts of my body rythimcally when I'm thinking too. NEver taken any anti-psychotics.

In your case, it's "cock jaw".