why would some guys do this?

there is this older married guy and his wife who was good friends with this young girl. they use to talk to her, laugh, ask her a favor or two. now suddenly he is not talking ot her, only his wife is talking to her. she wonders what did she do to him. when he does say something to her or even greet her, it is either for her to help with something very important, or he just say a very short and sharp sarcastic comment then walk off, never to speak with her for a long time and so on it goes. now there is this new lady that is a bit older than than the young girl. she seems nice, attractive. she and the girl becomes Friend. but then the guy and his wife started to make close friendship with this lady and the three of them is now buddy buddy. but the girl is still Friends with this lady. but whenever this lady has a problem, he is there to help her. he never did this for the young girl. although he tired few time to assist the young girl, but she never saw the need for his help. so she declined his help. but it seemed there was no problem with that. now more than ever, he is acting very strange towards the young Girl and more friendly with this lady, who has bonded with his wife. the Young girl lives alone and they never call her or pay attention to her as they do this older lady. then the Other day, he came up to the young girl and said something sarcastic once more, then put hi hand on her head slightly, then walk away.

can any guy here tell what could this man be doing?