When Chandler talks to Monica about all the jokes the friends have kept making, he reacts immaturely by trying to pass off their relationship as "casual". Monica gets angry with him and storms out, avoiding him when the two get within talking range.
Chandler's behavior is immaturely dismissive of Monica's feelings, attempting to downplay their relationship as "casual." Monica reacts angrily and storms off, avoiding Chandler when they are close enough to speak.