Both are astrology terms that have been adopted into popular speech. Both imply the motion of the stars/planets are the cause for something that happened. "Maybe the stars aligned" This phrase refers to the astrological belief that good things can happen depending on the position of the stars. This phrase is used as a comment when someone experiences something fortuitous or incredibly good. For instance: "How did she get that great job? I don't know, maybe the stars were aligned." "Maybe Mercury is in retrograde" I would say this phrase is less common than the above. The phenomenon refers to when Mercury appears to be moving backward (retrograde) in the sky. Astrologists maintain that new things should be avoided during these brief periods due to the supposed high rate of frustration and confusion. It can be used as in: "He fell down the stairs twice in one day? Mercury must be in retrograde."