The prediction of poor turn out for the event was self-fulfilling, once people heard the turn out would be bad, they didn't come so the turn out was bad.
If you describe a statement or belief about the future as self-fulfilling, you mean that what is said or believed comes true because people expect it to come true. Fear of failure can become a self-fulfilling prophecy. if a statement or belief about what will happen in the future is self-fulfilling, it becomes true because you expect it to be true and so behave in a way that will make it happen