Why would a guy act rudely- unbalanced to his ex?

after he dumps a girl why does he act rude to her!?? i mean why tease her or say her stupid and try to ignore her and avoid eye contact in a way that will make her crazy?? then just talk to her sometimes normally and call ask for a lesson or talk about the exam study papers?? why is he just not normal if he can be normal while asking for the study papers?? i mean if you are over a girl cant you be normal?? i am not giving any signals that he might think i still like him and freak out! it has been 2 months for gods sake!