Korry,
The 'browning' effect you describe is called 'bloom' by the industry. As the tire dries, a component intended to keep the tire from dry-rotting reacts with the air by oxidation. That causes the phenomenon you see.
I would bet that those that do see this occur may more often be people that wait to 'dress' their tires until they are completely finished with their detail. Had they 'dressed' the tires before they were completely dry, I'm betting they wouldn't see 'bloom' occur at all.
Cheers,
Ashley