Edit (I looked, unfortunately most of the sources are astrology, not astronomy), but Mars last went in Retrograde March 2014 and it's not due to go again till May 2016 (every 2 years, 2 months). My guess below was incorrect (though it's kind of cool, I'll leave the picture up).
Mars does have a tilted orbit compared to earth and that could be the reason. That and the angle from the earth changes too.
Mars, from our point of view, slows down, temporarily reverses direction then returns to it's original direction and continues on. The entire "S" motion takes maybe 3-4 months and your 2 photos are 24 days apart. (but this won't happen till May 2016)
Source of photo and explanation here: