Basic supply & demand. People were having to wait quite a while for their iPhone orders to ship. Mainly most new iPhone buyers seem to give their used iPhones to family & friends, so those people may be waiting for their hand me downs. Some of those hand me down iPhones die and eventually need recycling. When those family & friends used iPhones end up without an iPhone I suspect they purchase a cheap Android phone to tide them over. Hence the dip… It’s a theory off the top of my head, do with it as you please.