Why does my chihuahua scoot around on her butt sometimes? She holds ger back legs up and pulls herself forward with her front legs for a foot or so.

I would not forcefully say that it is for hygiene aspects that dogs do it but it might be a method of cleaning in order to get rid of odors that may attract other animals as well as enemies. So this activity could be based on instincts that remained from the wolves as I saw the same thing with different other dogs.

It can be to scratch it or because they are clogged but it don't always hurt tell the groomer bout it they push on their butt and it shoots out

Dogs  have  anus glands  where they're rectum is
and   sometimes  that  will get  irritated or inflamed
Sometimes  they  get  itchy  too , So yeah that is why they scoot