In high school, this girl in my grade was picking at her butt and was muttering something about "picking my thong out of my ass". I perked up and said "what?". She just sighed and said "nothing". I got some great whale tail sightings back in those days.
On this topic, I always loved those old Hayden Panettiere pics with her hand down her pants struggling with her thong.
It depends on what size you wear. If your thong is too small for you, the strap can cut deep into your crack. Also there is a lot of rubbing going on when you have a small thong on. I had this issue when I started off with women's thongs. They were too small for me, so I couldn't wear them for too long.
Also, I think chicks find g-strings uncomfortable because they cut deep. That's why they wear cheekies.
So with this new information, I've decided to edit my theory a bit:
People that wear thong underwear aren't bothered by it. You are much more likely to see a girl wearing granny panties (I consider any non-thong/string underwear to be granny panties. So this includes boyshorts, bikinis, hipsters, etc) struggling to fix their wedgie or complaining how their underwear is riding up their ass. The only time you'll see a thong wearer doing it is if she's just started wearing them, if her thong is too small, if she didn't properly place her thong in a comfortable position when she pulled it up, or if she's been very active and her thong is all sweaty. You're never going to see a seasoned thong wearer publicly picking her butt.