its a few factors in play, one is the normalization of far-right policies by the other big parties (SPD, Greens, Union) and their inability to fix the damage caused by the financial crisis of 2008, the rise of anti-migrant beliefs in the german working class, and a few others

i think this video explains well the rise of the AfD in germany, at this point its only a matter of time until one or more parties decide to make a coalition with the AfD