cuz usually that's the only way to get them up. haha I no it sounds weird but girls pants are usually tight and you have to move around to get them on.
our pants are made to be tight in areas where guys' pants arent. especially if a girl as thicker thighs (like me) you have to do it just to get them to go on easy.
The tight pants thing is really the key here. If you want to understand it, put on something tight. Do it with shaking and then without. You'll get how much easier it becomes if you shake it a bit.
our pants are more formfitting and so we have to move our bodies some to get into them. it makes things easier than just pulling them straight up. we don't do it just to do it
Their pants are tight, so they shake their pants, not their butts, to get the pants up. The butt shaking is a side-effect of putting on tight clothing.