This reminds me of a situation we once had with a Saint Bernard - we were at a strange house, and she followed us up a steep flight of stairs, and then was "stuck" and wouldn't come down. We finally realized the angle was so steep, her loose hide rolled forward and partially obscured her vision when she started down. You try coaxing 180 pounds of reluctance! We had to lead her down, gripping her extra skin gently and collar firmly. She never went up there again!

Does your dog go up and down other stairs? Maybe there is a lab-eating boogieman in your basement, and you haven't noticed!