It took me a little while to figure this one out.
To be honest, when she first started doing it, it scared the shit out of me. I thought maybe she was having some sort of seizure, or that she was in a great amount of pain, or that maybe I shouldn't let her watch Paranormal State with me anymore because I wondered if she had somehow become possessed.
But with time, I realized what the issue is. She does this when she's tired. Overtired. When she desperately needs a nap but is fighting it with all her might.
Sometimes popping the soother in her mouth will make it stop. Sometimes she requires that soother to be dipped in gripe water.
She did this in the bank the other day and the whole place went silent and people turned to stare. Understandably I guess, it sounds terrible.