What do the scholar’s suspect happened to the wine?

They believe the wine was spilled on purpose because Lord Asriel knew it was poisoned.

“Do you really believe he knew about the wine?” said the Librarian.
“Of course he did. I have no idea how, but he knew, and he spilled the decanter himself. Of course he did.”
“Forgive me, Master, but I can't help being relieved. I was never happy about the idea of...”
“Of poisoning him?”
“Yes. Of murder.”