I'm no expert, but it seems to me to be leg strength. I've done rides where my legs were so tired by the end that I just wanted to sit on the descents (and they don't make a sports bra good enough for that). It kind of reminds me of skiing - I think I'm in good shape until there's a really long run, and then my quads feel like they're going to blow up There is definitely balance involved too, as you want to use your weight to steer and that can be harder to think about standing.

As someone who started on a mountain bike, it never occurred to me that it would be difficult to get your butt off the seat going downhill, but I took my sister (a roadie) out last summer and she was very much like you. I finally got her to try it going down the street, but as soon as she was in the trees she had to have full contact with her bike. I didn't take her anywhere steep enough that she'd be forced to move her butt to avoid going over the bars, though.