Because of what I heard in the library. I didn't want to, and I don't know how. I just did.

I was working at a table with my arithmancy group, and I suppose I was fiddling with my quill. I don't know, I wasn't thinking about it much. But then I heard someone say that "Lestrange has -" well, something rude. About how my hands were "busy" with my quill and isn't Finch-Fletchley lucky.

But no one would say that sort of thing in the library. Not out loud where I could hear it.

So either I'm mad and hearing things, or. I'm hearing things.