By the time I quit a place, the reasons are well-known. I guess I'm lucky that way! :-D

Is "going back to school" a thing in Germany? From what I know about that education system I'm guessing not, but in the US that's a standard answer.

"I want to move to be closer to my aging parents." - might work. If you haven't shared much about your life, it's an easy lie. And it's dull and depressing, so the odds of followup questions are slim. I hear it a lot when interviewing job candidates in our area.

Because I don't work here anymore.

Why are you yelling?

But if you have legitimate gripes about them making you do their work why not tell them? You are leaving, it's not like they can do anything about it now, but if it's their fault (for lack of a better word) why not just say so?

Just tell them, "Bubatz ist jetzt legal!"

I signed an NDA

Personal obligations I can't discuss for the sake of someone else's privacy