Well, with regards to boob physics, if a woman is laying on her back, and the mass was perfectly centered and balanced over the attachment point of the chest, then yes, they would have a simple back and forth sway.
However, on a woman with natural breasts with some heft to them, gravity is going to generally pull them outwards to the side. Now, assuming a back and forth bodily rocking motion of, say, standard missionary, the attachment point of the boobs in relation to where their mass has settled (i.e. the side) is perpendicular to the rocking motion.
As the attachment point doesnt really move much (or else the boobs would relocate!), only where the mass has settled will significantly change, creating a pendulum like effect, which will create a natural arc motion.
Then, as the boob reaches the limit of the arc, enough of the mass has come back towards the center, and with enough inertia, they could just sway towards the center, more or less completing a full circle, and giving them the circular wobble they are known for.
So, yeah. Boob physics.