It's mostly fear after Chernobyl, fueled by environmental groups and the Green party primarily.

Weren't the anti-nuke hippies already quite a force before that?

Pete Seeger wrote his anti-nuke spin-off of "Acres of Clams" in the 1970s I think.

Not specific to Germany but there's been a very vocal anti-nuke movement almost since the beginning. They were pretty successful in ending new nuclear generating plant construction in the USA also.