It's easy to see how it would happen: A group of hikers makes it up Mist Trail, a short, shady, family-friendly trail that, in spite of being such an easy hike, leads to one of the most beautiful lookouts at Yosemite National Park, as well as one of the most beautiful swimming holes, Emerald Pool. It's got intense green algae (thus the name) and slippery granite sides, so you can spend hours goofing off like this. (That might as well be a photo from the Frizzelle family album—my parents loved Yosemite, loved the Mist Trail, loved the Emerald Pool.)
Emerald Pool and the edge of the waterfall are separated by a metal barricade and signs in multiple languages warning people not to climb over it, but after the hikers spent time goofing off in Emerald Pool, someone got the big idea to scale the barricade and get closer to that glorious lookout—after all the barricade is still a ways from the edge of the waterfall. But granite covered in algae covered in water is slipperier than an oil slick on an ice rink. Reportedly, the 21-year-old, 22-year-old, and 27-year-old "were in the water about 25 feet from the edge of a waterfall, standing, playing and taking photographs." Then: "One of the victims then slipped and fell, the second victim tried to rescue that person, and the third victim tried to save the other two."