"The worm has turned," is a saying that traces its roots to an old proverb: "Tread on a worm and it will turn." Its original meaning was that even the lowliest of creatures will react to harsh treatment. William Shakespeare makes use of this idea in Henry VI:
To whom do lions cast their gentle looks?
Not to the beast that would usurp their den?
The smallest worm will turn being trodden on,
And doves will peck in safeguard of their brood.
Today, the worm has turned is used to note how something has changed, a situation has reversed. He who was once on top is now on the bottom -- or vice versa. And in sporting events it is used to indicate a complete shift in the tenor, or momentum, of a game.