Why would he go quiet when I walk past and then watch me from a distance?

My crush has been acting really weird around me, he is a very outgoing and alpha male guy. Tall, dark hair and handsome and he can pick whatever woman he wants.

Yesterday, when he saw me, he was talking with his friends, as soon as I walked past he stopped talking and then kept watching me as I past him. When I caught him looking he turned around and walked away.