Although Karenin found nothing improper with his wife and Vronsky sitting privately, he realizes that everyone else thought it was peculiar, and so he resolves to have a talk with his wife. Karenin is not a jealous man, but for the first time, he begins to realize that there is a possibility that his wife is falling in love with someone else, and he is horrified.
Karenin only thinks that there may be something improper in the relationship between Vronsky and Anna after realizing how the rest of society perceives the situation. He is not by nature jealous, but he is very conscious of how others perceive him.
Karenin paces back and forth, trying to figure out what to say. He composes a speech as though making a political resolution and cracks his knuckles. When he hears Anna arrive, he is pleased with his speech but nervous about the talk he is about to have.
Karenin treats his personal relationship with his wife as a business matter to be dealt with. The tic of cracking his knuckles indicates his physical discomfort and his desire to maintain control.