High traffic on stairs causes carpeting to wear out and loosen more easily than on floors and low-traffic areas. Incorrect installation and choice of the wrong carpet may also cause carpet to loosen easily on stairs.
Remedy loose carpet by replacing it entirely or simply reattaching it to the stairs. Stretch loose carpet back into place using a carpet stretcher to hook the bottom of the carpet back into the tack strips at the back of each tread.
Until you can rent a carpet stretcher or hire a professional, create a temporary fix to prevent falls on your stairs. Simply stretch the carpet by hand and staple into place with a staple gun and construction staples; place staples at the back or ends of tread to avoid stepping on exposed heads. When you can permanently fix the problem, use pliers to lift and remove the staples.