I'm going to guess the person suffers from vertigo, as have I to a small degree - particularly on the metro escalator in Rosslyn, VA (across the river from DC). The sensation occurs going down or up very tall escalators in a tunnel. When it hits, you feel like you are traveling horizontally with some weird tunnel vision. This is terrifying and can cause you to feel like you're falling - even when you know you are going down or up, your eyes are telling you you're traveling horizontally.

I've also gotten this driving a car through long tunnels as well, going down or up (Baltimore 895 harbor tunnel can do this).