A) Germany has a (predominantly) proportional system, but parties get in on list votes only if they pass 5%. Seing its poll numbers sink, FDP campaigned for CDU voters to give their list vote to the FDP - maybe with overwhelming success.

B) Last night, popular German talk-show host Stefan Raab did his own election show. I don't have a very high opinion of him, yet he is very popuzlar among a certain section of the youth. Meanwhile, in recent years, the FDP has tried to attract just this clientele. Yesterday, in the show, they held a mock telephone voting of viewers, and the FDP of course did rather well. Maybe this had a lasting effect on some young people who would have voted for CDU.

C) IMO most likely: on the TV channel ARD, polls showed a dramatic swing in preferred coalitions: the popularity of a CDU/SPD Grand Coalition slumped, that of a CDU/FDP one rose. Maybe a lot of CDU voters wanted to prevent a Grand Coalition.

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