Its like i dropped off the face of the earth! why does he ignore me?

So there is this guy that I really like and from, I would say the second week of school or so, I've been talking to him. We became decent friends overtime. The past week, he has just been completely disregarding me. Like I said before, it's like I dropped off the face of the earth. We don't talk in class, in the halls, he's been taking a different route, and when I pass him, he looks at me and goes back to normal conversation with his guy friends. I don't know why he just ignores me. Its kinda like when in those movies, the one girl friend just ignores the other girl friend, and a week before they were both friends. I know for a fact that there is no reason for him to just suddenly avoid me because during this month, we hardly talked at all, so it's not like I would annoy him or whatever. It would just be like, a casual whats up or a wave in the halls, but now its just nothing. I overheard him and his girlfriend just broke up too in almost the same way we stopped talking. My friend said that she said he just left her one day being all "I'm not into you anymore" and doesn't talk to her. I'm just confused why he is being like this?