Ok, the problem might be a few issues. If you were using a 10" buffer, you probably werent really getting the abrasives in the polish to break down and effectively do what it needed to do. Therefore, my guess is the paint is still in far from perfect condition. Those giant polishers are more for spreading wax than any effective correction work.
Secondly you parked under a tree. So as it was raining, the rain was washing all the stuff that the tree has been collecting off and onto whatever was underneath it (your car).
If you are trying just to get some of the oxidized paint off and get it somewhat shinyer, you might want to just try a cleaner wax. One step, ok results, and you are done.
When it rains, and my car is not underneath anything, it does dry relatively clean. There will always be some water spots, but they usually come right off (regardless of whats on top of my paint). WGDGPS I do believe has a longer curing time than most sealants, so its also possible it had not completely cured in 12 hours, especially in high humidity.
More often than not, its more about the process than the products themselves, and perhaps you arent using the right products and process to achieve the results you are hoping for.