I am guessing that it was a pretty steep grade and it had already downshifted from D to 3rd on its own. 3500 rpm at 80 mph in 3rd may not be unreasonable, but I'm not sure.
I remember some of those grades from Las Vegas back to LA years ago and I'm not too surprised at what you experienced. I guess my thought is why wasn't 80 mph enough for you.
