Wow, that's pretty crazy. I'd speculate the fish washed down from one of the streams in that area that holds or is stocked with trout (Antietam?). It's a miracle that guy was still kicking at all, because the Potomac near DC has been up to 90 degrees recently.
I can't find the posts now but we've seen a few reports on here of people catching rainbows near Chain Bridge and Rock Creek in the Spring, but nothing nearly this late in the year.
Dalton