Mechanism of antiperistalsis
What is the mechanism of antiperistalsis that occurs during vomiting?



Emetic agents in the bloodstream stimulate chemoreceptors in area postrema and nucleus tractus solitarius [1]. 
Nucleus tractus solitarius is connected to motor neurons in ventral medulla and hypothalamus [1].
Esophageal and gastric peristalsis is inhibited. Small bowel peristalsis is reversed. Esophagus and stomach don't have antiperistaltic movements [2].



  Why the peristaltic waves normally don't propagate in anal to oral direction?


Because of the mechanism of descending inhibition. Distension of the esophagus and intestine causes a reflex action that leads to circular muscular contraction just above the bolus/chymus. Longitudinal muscle also contracts (shortens) [3].





Reference:


Hornby PJ. Central neurocircuitry associated with emesis. Am. J. Med. 2001 Dec 3;111 Suppl 8A:106S-112S. PubMed PMID: 11749934. 
Brizzee KR. Mechanics of vomiting: a minireview. Can. J. Physiol. Pharmacol. 1990 Feb;68(2):221-9. PubMed PMID: 2178746. 
AUMSA. Digestive Physiology. May 2012. CC-BY-SA-3.0