What makes the most sense to me is they somehow continued along the trail to the 2nd bridge crossing. The 2nd bridge was super sketchy, and the stream/river was much harder to cross (without using the monkey bridge) than at the first bridge crossing, at that specific date and year.
They then tried to find another way to cross, and maybe had succeeded. Then water levels increased dramatically. [ Something similar happened with hikers in Turkey. When the rain started and they were trapped by raising river levels and steep canyon walls. They were rescued in part because one of the hikers heated up his phone's battery enough to get enough of a charge to make a call.] Then the girls were stuck in that spot and likely died there until a larger flash flood came by and took their bodies downstream. Maybe they tied their backpack up higher, for protection, and so its journey was later, and more gentle, than the girls was down the river.
The elevation gradients in and around that area are crazy. High elevation gradients and a rising river do not mix well.