It seems to me that it is probably due to people scuffing their feet along the step as they walk up the escalator. It just amazed me that it could make such a difference, but I suppose there must be hundreds of people using that escalator every day.
I checked the escalators at London Bridge on the way back, but there weren't any stripes, but then I'd expect London Bridge has slightly fewer people through it day (still a considerable number though), so now I'm wondering what the minimum requirements are for the stripes to appear.