Why is it "vaguely ominous" when Jotham Powell refuses Ethan's dinner invitation?

Powell never turns down dinner...... Ethan wonders what could have happened to cause his refusal. Truning down dinner was completely out of character.

To Ethan there was something vaguely ominous in this stolid rejection of free food and warmth, and he wondered what had happened on the drive to nerve Jotham to such stoicism. Perhaps Zeena had failed to see the new doctor or had not liked his counsels: Ethan knew that in such cases the first person she met was likely to be held responsible for her grievance.