The most common occurance of shomitting usually happens after or during a heavy night of drinking. The excess alcohol causes the victim to power spew. As the abdominal muscles contract in an effort to expell all your stomach contents out your mouth, unfortunately this abdominal contraction also encourages any poop chambered in your chute to also be expelled at the same time....
Can also occur if you get food poisoning and suffer from IBS.
Jenny: I need to spew
John: I'll hold your hair back
John: Holey shit, did you just shomit....that's disgusting.