I've also hiked in this area (love it) and wondered about those old cars. My husband was absolutely fascinated by them, and tried an on-line search for some info but came up with nothing.
The only thing we could think of was the old tourist railway line that used to run along the gorge bottom. A portion of the gorge trail actually follows the rail path. Perhaps after the rails were removed and before the area became impassible, these old cars may have been driven down & abandoned.
I don't suppose it's possible that they actually went over the falls! They seemed nestled too far up the gorge wall for that, but who knows?
I must say that it captures the imagination much more to picture them being pushed or driven off the edge, hurtling through the abyss, bouncing end over end, to ultimately come to permanent rest on the gorge floor...for future generations to explore and wonder, how did they get here?