Another possibility. Myself, I'm inclined to believe they met their fate the same way most of the other extinct megafauna did in northern and western hemispheres. Fact is, there have been indications from other parts of the world where tsunami type detritus was mingled with volcanic ash, mangled trees and the like -- along with sand and gravel -- from somewhere.
The end of the Pleistocene was not a pretty picture. Our most recent and probably least studied extinction event on the planet. Too close for comfort maybe.
I suspect this is a thanatoscenose assemblage of Pleistocene megafauna. They probably died due to a large rain (flood) and their bodies washed down into a common chokepoint on the river, the bodies decayed and ended up on some riverbottom. With time the channel of the river changed and the bones became the property of an oxbow lake which finally filled in through eutrophication. They laid there until they were found.