Honestly, as far as explanations go for why you'd find a land animal so far out to sea, getting caught in a flood is not a bad thesis. Consider what must have happened to an immense number of fauna when glacial lake Missoula blew, for example! But the fact that this is the only one of its type found so far in the area does not speak for a large one. Something as simple as dying while fording a river is another possibility.

https://en.m.wikipedia.org/wiki/Lake_Missoula