“The outer fruit layer, left on black pepper, also contains important odour-contributing terpenes including pinene, sabinene, limonene, caryophyllene, and linalool, which give citrusy, woody, and floral notes. These scents are mostly missing in white pepper, which is stripped of the fruit layer.”—Wikipedia, peppercorns. So I would guess something in all that doesn’t smell great when soaked in water. Also if you read about black pepper, white pepper is basically soaked black pepper—the fruit comes off and the dry seed it used, so that is another possibility.