Leggings can slide down for a number of reasons:
Your leggings are the wrong size. They are simply too big for you. If you feel it too loose on you, it could fall down easier. Leggings should never be loose at your knees. If they are, you need to size down.
Sliding down could also happen when the size is too small. Being skintight, leggings could slide down when being stretched too far during workout. You should size up one size or two in this case.
Sometimes it depends on what types of leggings you are wearing.
Sliding down generally happens more often with standard leggings. Seamless leggings has way better "stretch" effect. A size S can often fit a normal M, sometimes even a size L, in standard leggings.
Leggings won't stay up when the elastic in the waistband is bad quality or has worn out.
Quality seamless leggings that has a thick waistband or tight knitted waist will significantly reduce chances of falling down. TEFIN3 or Gymshark seamless leggings are those of my favorites with excellent tummy control around the waist.
