There is no pre-set order of resort drop-offs. There's not even a pre-arranged grouping of resorts that always goes together on the same bus. It's all arranged on-the-fly, based on what guests are coming in at what time and going to where. If, by chance, 50 people are arriving at MCO around the same time and are all going to POR, then they might have a bus earmarked just for POR (just an illustration, not a realistic example).
As a general rule, for any particular bus, the resort closest to MCO is dropped off first, etc.