vomit is usually liquid or heavily digested stuff. feces is compacted and excreted. my imagination maybe a little specific here but so is the description "fully formed feces"
When there's no place for the compacted feces that's building up to go (due to a failure in peristalsis, for instance), it backs up into the the stomach, which naturally irritates the stomach and causes the person to vomit the contents, which includes their feces.
