It began in the early 2000s with the government of Chancellor Schroeder. To the shock of the far-left wing of the Social-Democrats, he was far more business-friendly than expected.
In 2007, the far-left wing of the Social-Democrats seceded and joined the socialists of the PDS, forming a new far-left party, "The Left".
Then the Social-Democrats joined Angela Merkel's coalition-government as the junior-partner. She implemented some policies of the Social-Democrats and stole their credit for this. With that, the Social-Democrats suddenly looked neutered and useless to the voters.
Then came the immigrant-crisis in Europe. The Social-Democrats tried an inoffensive middle-of-the-road approach, and again the voters punished them for being useless and toothless: The Social-Democrats lost again voters to the Greens and the far left.
And then came last months EU-election: The big topics were immigration and global-warming and again the Social-Democrats had nothing really exciting to offer. The big winner was again the Greens.