Usually used in context of a video game: To either accidentally or on purpose push or deflect another player, causing them to fall off a ledge or platform, miss an intended objective, or fail in any other way. Popularized by Gavin Free of Achievement Hunter.
Ohhh, sausages! I would've made that jump, only Ryan bunced me just as I was about to land!
It's cockney (i.e. from London) rhyming slang for earning money. Bunce is derived from bunsen burner (a common piece of school laboratory equipment that produces a single open gas flame and named after Robert Bunsen) and this rhymes with earner.