When the game respawns you in Arcadia, I think it picks a random point in a small area. When you walk up into the Pillar Mountains, it puts you slightly on the stairs for an instant relative to this spawn point. It then clips you into the nearest area the player is allowed to stand on. It turns out that if your respawn point is low down enough and far enough to the right, on one certain floor layout (excluding trees, rocks and exits, just the shape of the grass) the closest place the player can walk on is actually out of bounds.
Your respawn point is only re-allocated once you've walked into a new room. This means you can always get out of bounds if you kill yourself while out of bounds. This can only be rectified by saving and quitting to title, or by walking into a new room if you happen to not be out of bounds.