I cannot tell you where exactly this stems from in law, but it is roughly the same mechanism as for inventions. If you create or invent something outside of you

I cannot tell you where exactly this stems from in law, but it is roughly the same mechanism as for inventions. If you create or invent something outside of your regular work that may be relevant to your employer, you have an obligation to inform them and offer them rights to it. There are several sources out there (in German) that describe this.