Apparently it's more because of inertia and the rotational spin of The Sun affecting the entire solar system, than anything else. Which actually makes me wonder if Voager and Voyager II will have issues once they fully pass beyond the heliopause.

Apparently our star, and therefore the rest of the solar system, moves around the galactic disc in the direction of galactic spin, but it wobbles "up and down", as well as possibly "left and right" as it orbits the galactic center every 225 million years, or so.

The reason that Voyager, Voyager II, and pretty much every single other probe we send outwards might have some issues once they pass the heliopause is that our solar system is a bit tilted compared to the galactic plane of rotation. They may encounter some background inertia that we didn't account for.