Question

how does food move in the opposite direction when we vomit

Answer 1

Because of the negative prestaltic movement of the alimentary canal.

Answer 2

It usually occurs as a precursor to vomiting. Local irritation of the stomach, such as bacteria or food poisoning, activates the emetic center of the brain which in turn signals an imminent vomiting reflex. Food then moves in the opposite direction, often from the duodenum into stomach.