Yes, the vortex rotation of these galaxies you describe, is a bit strange. Have you noticed - that the best, clearest example of this type of rotation appears to be associated with the galaxies classified as ... Barred Galaxies. Go look up pictures of this type of galaxy. (They're very beautiful, photogenic.)
Now normally this rotation would be classified as a 'Rankine Vortex', with an inner, central Forced Vortex rotation component, and a second, outer, Free Vortex Rotation component.
In these galaxies, the Forced Vortex rotation is as expected ... but the Free Vortex rotation section does not have the correct rotation velocity signature. Instead of having the normally expected, 2'nd order, power decay equation/line ... the decay is almost non-existent - almost horizontal.
So, it is really the outer portion of the vortex that is not performing as expected. Its orbital velocity is way too fast - for the amount of visible matter present.
Astronomers came up with a solution to this problem - called 'Dark Matter'. It has real magical properties.
The Forced Vortex portion of the Galaxy's vortex, has another Physical property that must be considered and looked it. The Forced Vortex 'conserves angular rotation' ... the old picture of the beautiful ice skater speeding up or slowing way down depending upon if she extends her arms outwards or pulls them vertically up along her rotating z-axis as close as possible.
What this means is that a Barred Galaxy is probably not operating in steady-state mode. Instead, it is slowly decaying ... which in this case means that every time the central Black Hole swallows and eats one of its orbiting stars - the Forced Vortex, to conserve angular rotation ... ejects a star or star mass, off of the ends of the central Bar-of-Light.
This conserves the angular rotation rate and energy of the Forced Vortex. Meanwhile, the ejected star(s) fly outwards. Then, according to orbital rotation mechanics - flow backwards.
Look at the pictures of Barred Galaxies again. See this "flow of stars" ... as 'two', beautiful, spiral arms ... One off of each of the two ends of the Light Bar ... as the stars flow upwards and backwards.