As one who worked in Yosemite for nearly 10 years, has hiked every trail leading out of the Yosemite Valley and countless miles in the back country, and has heard several accounts of waterfall deaths:
At Vernal Fall, the victims don't usually fall from the Mist Trail but from the pool at the top of the trail, top of the cliff, behind the falls. Everything is safe enough until you go over the rail, too far into the pool, too close to the falls. The surface of the pool can at times be glassy, looking very calm. The appearance is deceptive. That water is moving very rapidly, although smoothly, toward the cliff. The rock floor of the pool is mossy and slippery.Once your feet are swept out from under you, you're headed for the fall. There's nothing to grab to save yourself. There's no stopping. Say Goodbye. You're done.
At the base of Vernal Falls is a deep and turbulent pool. Bodies are sometimes trapped under or against rocks and don't resurface for months, if ever. Search crews must wait until the seasonal flow reduces enough for them to enter the pool. Other times the bodies are circulated violently, like in a washing machine, beaten by the falling water until they are pulverized and the parts wash downstream, and there's nothing left to find. Right now the crews are probably not looking for a body, but for a torso, some limbs snagged on a log or rock at the water's edge, or a skull tumbling downstream.
Several of my friends were on the trail maintenance crews and the Search & Rescue teams and I've heard their stories. The closure of the trail is standard procedure. They say its for the public's safety, and to some degree it is. The most likely reason is that they don't want to have to work around gawkers who will invariably bunch up on the trail to watch, get in the way, and ask questions all day.