Kaa is threatening Bagheera, who has just smacked the python against a tree branch. It was the panther's quick reaction that saved Mowgli's life. Just like the boy before, now Bagheera is being hypnotized.
I remember watching this scene in a movie theatre -ages ago- and the strange feeling I got from seeing Bagheera's eyes become as large as tennis balls. It is an exaggerated effect to show that he, too, can't avoid Kaa's hypnotic stare.
Bagheera is a relatively realistic character, with proper big cat anatomy and motion. So you don't expect such a cartoony expression at all. But it sure lets the viewer feel what the panther is going through in this situation. I love this scene, animated by Milt Kahl. These are copies of his rough animation drawings.