The seven-year-old, who is trained by Willie Mullins, powered down the home straight and seemed to be heading for the anticipated routine victory, but she clipped the final fence, which sent her and Walsh crashing to the ground.
"I don't know why she did it. She was a bit far away [from the fence]. That's racing and that's what happens. At least Annie Power got up," Walsh, who has won three other races today, told Channel 4.
Although the fallen mare will still be assessed, it does not appear that a significant injury has been suffered, while Walsh also confirmed that he was fine.