When a routine infield pop-up is anything but routine...

Infield fly only applies if there’s a force at third- either runners at first and second, or bases loaded. It’s designed that way in order to prevent the defence from turning a near-guaranteed double play on the pop-up (if the runners stay, they drop it and turn two on the force with those runners; if the runners go, they catch it and turn two or three on those runners). In this case, there’s no possible real advantage gained by the defence in letting it fall, as the batter-runner is assumed to have gone to first, so there’s no potential double play.