I also put my fingers through my belt loops, tug up and do a little wiggle at the same time. Especially when I’d go to the movies with my ex because he always insisted sitting close to the front of the theater. I’m not about to stand up right as the lights come up at the end of the movie, in a huge room full of people, while my underwear are showing. Especially because I do tend to wear thongs. I like them, but that doesn’t mean I want everyone else to know I wear them. (Besides right now, because it’s relevant to the question, so just shut up already, please!)
But yes, it’s because of hips. If women aren’t rock hard we tend to squish a little bit in that area, while wearing jeans. If they aren’t the absolute perfect fit, which is very hard to find, they’re going to come down a little bit as you sit and stand.