First and foremost I would like to say my thoughts and prayers go out to all those involved in the incident.
Next, I know this ride inside and out. Two weeks ago, I wascoming back home from nine months on the career start program. I worked at Kali River Rapids for those nine months. Let me tell you it was very weird to see this story all over the news. Now in all 100% honesty, I have not been in contact with any of my coworkers yet but I can let you know what more than likely happened based on what I've read on this thread.
Now not to sound like a total CM, but Disney doesn't evacuate guests, we exit them. Anyways, my thoughts on what might have occured that morning: Normal day, Kali probably had a few RIDE STOPS( just stops the rafts where they are) this can happen for a number of reasons. The sensors on this attraction are VERY sensitive, a little anole(lizard) has been known to cause an E-STOP(stops the rafts & drains the water) so more than likely a few RIDE STOPS then the computer not knowing the specific problem, issued an automatic E-STOP. At this time an In-Show Exit must be performed. There are technically 4(four) locations to exit guests: The burnzone(fire spot), Elephants(by the exit bridge), turntable(loading platform), and the Up-Ramp(lift hill). To exit guests on the up-ramp a device simply known as "The Beast" must be used. Now "The Beast" is a large heavy steel contraption that will slide all the way up the up-ramp. Now what might have happened was a raft was stopped fairly high up the ramp.(It does take two cast members to push "The Beast") the CMs probably pushed it up and locked it into place. However it might have malfunctioned and slid down the ramp after exiting the five guests off.
Again this is just a guess a very detailed guess lol but a guess. Hope it helps those having trouble understanding.