Are you sure it was 'turnip' and not 'turn-up'? It is often said of anything surprising..."That's a turn-up for the book!" The 'book' in question is the one made by book-makers (bookies) on any horse-race and including all the runners' names and their odds. If a long-shot wins rather than one of the favourites, people say what I wrote above...in other words, the 'book' has been turned on its head, as it were.
Just a thought, as turnip/turn-up could easily be misheard.