UV brittling is definitely in play, but the tire just had such a sharp trimline at each side (well, as best I could see from behind it in stop-and-go traffic). The sun exposure would grade more evenly there I figure.

Precipitation, that would bring gravity in to explain the sharp top/bottom effect. But sprue-stripping precipitation? Freeze-thaw something something maybe?

How about this, the sprues *are* brittle all over the tire where sun reaches, it's just that it hailed this one time and and knocked off all the ones on the top, and nothing has knocked off the ones on the bottom yet.