The brand collaborations that explode online usually amplify something consumers already love. Walk through the snack aisle lately and you’ll see products that look like high-speed shopping cart crashes. Skippy peanut butter inside a Milk Bar dessert pie. Or Pop Secret popcorn paired with Kraft cheese flavors. A few years ago these mashups might have felt weird or gimmicky. Now they regularly generate headlines, social chatter, and lo-oong lines of curious buyers. The obvious explanation is...