http://io9.com/342520/when-galaxies-col ... each-other

Scientists still do not understand how the galaxy got its oppositely oriented arms. One possibility is that the inner arms are the result of a struggle with a smaller galaxy that veered perilously close to NGC 4622 and was swallowed. Before being ripped to shreds, the smaller galaxy could have stirred up matter in NGC 4622's inner regions, leading it to settle in a spiral pattern opposite to that in the outer regions.