Well, that's a way to say it. At least Will is blunt about it, but the surprise is still there. probably it would be there regardless of the way he said it, considering it's not an easy subject to digest (aha)

Kaneki is actually glad he put the cup down because Will would have noticed his hand shacking slightly at such confession. But his face betrays him, anyway. Kaneki's eyes go a bit wider too, because of all the things he didn't expect that. Silence also follows, and perhaps a bit of confusion, but he says nothing for now.

First, Kaneki thinks about Will's words. He wasn't aware. BUT. But what? And why? Who fed him cooked human flesh= are you even considered a cannibal if you were tricked or forced into eating such a thing? well, nuances aside, Kaneki can't make much of anything without any extra information.

Explain.

No, that sounds like he is demanding. Kaneki isn't demanding, specially because Will didn't even have to share this.

"Why did it happen?"