I hope someone with physics or mechanical engineering expertise will chime in.
My best guess is that it flipped and slammed into a rock, causing the buckle.
Alternate guess is that two guys overloaded the boat and got in, causing the buckle.
That's my guess as well. The boat would have hit the rapids, turned, flipped, crashed, buckled and eventually ended up on the shore. It would not buckle from the weight of two people.