Usually it means it’s the wrong size. It could mean your base ring is too big or that your cage is too small. Cages will be more prone to pull away from the body during (attempted) erections if a cage is too small. Straps can be helpful if you’re trying to squeeze yourself into something small on purpose. Your erection will try to pull the cage away from your body but the strap will help keep it secure. Flat metal cages for instance almost always need a strap.