They had the legal form of a registered association [0] and the ministry claims it went against the German constitution:

Associations whose aims or activities contravene the criminal laws, or that are directed against the constitutional order or the concept of international understanding, shall be prohibited. [1]

[0] https://en.wikipedia.org/wiki/Registered_association_(German...

[1] https://www.gesetze-im-internet.de/englisch_gg/englisch_gg.h...