No one noticed that despite being an impossibly powerful artifact, the Ahn'Ger Stone was not invulnerable. It was still simply a gemstone, just one with otherworldly powers that could bend and shape reality on command. And when Bill's attack on Gloria caused her to fling the Ahn'Ger Stone down the hallway, it had cracked. In the heat of the moment Bill had no idea that the artifact in his hand was damaged.
With his wish, the power coursing through the stone was not released safely or in a way that obeyed the rules that bound the stone to it's purpose. Not even it, if the stone did or did not possess such an ability, could guess at what would happen when Bill's wish was granted.
Whatever Bill wished for next was going to go utterly, horribly wrong no matter how he said it.
"I wish..."