Today, the Nasdaq Composite looked like a V: down more than 4% by lunchtime, closing off just 1%. Nobody knows exactly why. Around noon, the AI jitters came back, and traders dumped the highest-beta names—the frothiest, most volatile stuff—like Strategy (MSTR), the leveraged Bitcoin vehicle that had popped Monday; AppLovin (APP); and photonics maker Lumentum (LITE).