If the person is wearing a miniskirt, there may be an opportunity for an upskirt glimpse. There are better opportunities if the person is several steps ahead of you, hence above eye level. However, on crowded stairs it is then likely that other people will obstruct the view. If you look up a flight of stairs and someone is walking down towards you, then again there may be an upskirt glimpse.
If the person is wearing leggings, a tight miniskirt or tight jeans or shorts, then the act of bending the leg necessary in walking upstairs emphasises the bottom. It increases the likelihood of seeing VPL, as the material in the outerwear tightens, or whale tail, some bare back or even rear cleavage as the hemline tends to move downwards and the hem of any blouse or top moves up. Leggings may become somewhat transparent over the bottom as they stretch.
On some staircases, modesty panels have been added to stop upskirt views by people standing next to the stairs.