Why is he treating me this way?

Look, it isn't the why that you should question, it is the fact that he is doing it. Guys don't overthink things like we do, so if he is ignoring you, walking away and treating you with less respect than he once did, he is either toying with you, or trying to get you to be angry at him so you will leave. When guys act this way, back away, far away. Guys may not overthink, but they will become deceitful, they will become elusive, they will act out, and he is acting out that you aren't important enough to keep his attention. It sounds like he is on the prowl for someone new -- or, and this is where guys get weird - or he likes you and is making all this nonsense stuff to get your attention, to see how available he is. Either way, pull back and don't take it personally - like I said, guys don't overthink so it would not even enter his mind that he is hurting you, his friend. Once you pull back and leave him alone, just keep an eye on him, see how he acts - if he goes away, then he wanted to ditch you. But, if he comes back at you, there may be some interest there. Give him the stage all alone to act out, don't be a participant in his little routine.