My educated guess has always been that the term originated from the term “drop shipment”. Items delivered directly from the vendor to the retailer. Most media (cds, dvds, video games) that have a street date are delivered this way. Other normally stocked titles are replenished by the retailers central warehouse.
So when someone asked “When will U2’s new CD be available?” a clerk might answer “They will be drop shipped here on Tuesday.”
Artists later picked up on this and started the whole “My new album is dropping on July 7th.”
This has always just been my best guess but nobody has ever challenged it or found a better explanation. Is there another explanation?