It's really hard to know what happened. Press reports are pretty unreliable I think mainly because a lot of journalists don't really have a feel for the outdoors. Some reports even describe the two as "hikers" when it's fairly clear that they were in fact climbers.
There was a pretty big thunderstorm that hit about the time of the accident. That may have played into the accident either by dislodging the putative "refrigerator sized block" or by causing them to rush, "let's get the heck out of here," which could have in turn led to a mistake. I of course really don't know.
I understand that RMRU was on site at some point. I hope they'll publish a post mortem.
HJ