Jordan and Tatiana have brought some zouk influence into WCS over the years, and vice versa, as they have a big presence in both worlds. I think what you see is mostly an influence of slower contemporary movements and latin rhythms present in the music as David mentioned. There is more slow music played now for WCS, which allows for bigger sweeping movements. Blending the styles, smooth into sharp also creates more contrast and impactful moments in the dance. There are other proponents of mixing the styles as well, I could name names.
For me Zouk and WCS danced to slow music are both really mostly contemporary dance done with a partner, and it makes sense that they look a little similar, because a lot of it is foundational isolations, body movements, waves, ripples, off-axis turns, and the like. In contemporary dance, solo dancers send different blocks of weight in different directions to create counterbalance, and in partner dancing, we can exaggerate or enhance that since we can rely on a partner to create even bigger shapes and counter each other. And yes, we see that contemporary influence in ballroom as well.
And to be fair, sometimes it's dances borrowing from each other, and sometimes, it's just exploration of movement to similar music creating convergence.
