It wasn't a single trend, nor a monotonic one.

• Right after the 2009 elections, the SPD was shedding voters, the Left Party was overtaken by then.
  
• The FDP meltdown started at the end of 2009.
  
• Stuttgart 21 protests boosted the Greens above 20% in autumn 2010, but that melted away again.
  
• Fukushima put them closer to 25%.
  
• Several problems peaking in the debacle in the Berlin elections (commonly explained with supporters being scared away by allusions to a CDU-Greens coalition option) cut support back to the 15% region. After Berlin, the Pirates exploded federally, taking from the Greens and the Left Party.

Also note that there are simultaneous movements, e.g. a decrease in FDP and increase for the Pirates could mask a combination of FDP -> CDU -> SPD -> Greens -> Left Party -> Pirates movements.

_{*Lunatic*, n.
One whose delusions are out of fashion.}