Material stripped from the galaxy during its collision with a smaller galaxy (seen in the upper left corner of the larger interaction partner) forms a long tidal tail. Young blue stars, star clusters and tidal dwarf galaxies are born in these tidal debris. These objects move in a common direction within a plane defined by the orientation and motion of their tidal tail. A similar galaxy interaction might have occurred in the Local Group in the past, which could explain the distribution of dwarf galaxies in co-rotating planes.
UGC 10214 ("The Tadpole"): http://www.spacetelescope.org/images/heic0206a/
Credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA