First, Galaxies collide and collide often with other galaxies or dwarf galaxies (some even wonder if Globular Clusters aren't some kind of galaxies too). Maybe this reverse rotation is induced by encounters in the past.
I remember reading something about Galaxies turning around themselves in the wrong way. That was years ago. I don't think it's an exception.
But it might not be as astonishing as why spiral galaxies keep their spiral shape, even though they have (like ours) spiralled about 50 times for the past 13 Billion years. Normally, the outer stars must orbit the Galaxy's center at a much lower speed than those near the center, and yet...However, it appears every star orbits its galactic center at the right speed (in average) but it reaches a density wave (the spiral arm), slows down like cars in a trafic jam and mixes with the other stars within the arm, then, leaves the arm and regains its normal orbital speed. The density waves have a spiral shape but not all galaxies are spiral; many are elliptical or irregular. Could it be that density waves have a motion that is not necessarily related to the shape of the galaxy? We don't know.

Raoul