Schwalbe got back to me within three business days. Their warranty rep wrote:
" I’m wondering if the duration of storage had dried the tire a bit causing this split. In any case, it’s not normal or something we see often."
He sent me a coupon for a free tire, with free shipping.
I store my spare tubes and tires in closed plastic bags to slow the drying - out process (in an apt., so no climate extremes). It's worked for me until now. Having been kept in a closed plastic bag, the rubber doesn't look dried out to me, but rather brand new. But I'm wondering if some chemical reaction could happen, in an airtight enclosure, that would affect the rubber negatively. Anyway, I'm impressed with how this was handled by Schwalbe.
Thanks for the great idea, Bruce!