Almost certainly sunlight. The black is perhaps the bottom and the oxidized part the top? Maybe in a rack? It's not too badly oxidized. Oxidation of vulcanite is inevitable, but you can slow it down by limiting exposure to light and air. Keep it in a darker spot for the light and use mineral oil or Obsidian oil or Beeswax chapstick to limit exposure to air. Removing the existing oxidation is not hard. There are numerous thread covering the myriad methods for doing so.
I agree, regarding new pipes and even estate pipes bought from dealers. I expect the stems to be free of oxidation at time of delivery. Even dealers that offer to buff the stem before delivery sometimes do shoddy work. I have sometimes toldl local retailers whose display cases have numerous pipes with BADLY oxidized stems that if they don't sell a lot of pipes, be sure to stock only pipes with acrylic stems. These dealers don't know how to remove the oxidation and some don't even know they're oxidized. Worse, even when they know and can't or won't do anything about it, they don't reduce their already high price. The result? The pipe oxidizes some more waiting for a sucker to buy it.