Karenin was okay with Anna and Vronsky sitting together, but he has noticed that everyone else thought it was strange and improper. He's now starting to feel weird about it and decides to mention it to Anna.
After reading for a while, and following his normal routine, Karenin resolves that he needs to speak to his wife. Instead of thinking about the usual official business things he thinks about, he considers what he should say.
Karenin is not a jealous man; he believes in trusting one's wife, and has always had faith in Anna. For the first time, comments the novel, Karenin begins to imagine that his wife has an internal life, with thoughts, wishes, and feelings that he has never known.
Now, for the first time, Karenin has to confront life, and confront the possibility that his wife is in love with someone else.
This realization horrifies him.
He paces back and forth, thinking about what to say and what to do. He's confused, because this is really the first time he's had to deal with life's complications.
He begins formulating what to say to Anna as though he were composing an official report. And he's irritated that these "domestic" matters have to take up so much of his intellectual powers.
He cracks his knuckles.
Anna comes upstairs.
Although he is pleased with the speech he is prepared, Karenin is frightened of the talk he is about to have with his wife.