I am still suspicious that this has something to do with the relationship between Springer-Verlag and the Max Plank Digital Library (MPDL) which supports open access.

In 2014 MPDL purchase 110k out-of-print and historically significant titles. In 2015 Springer acquired open-access journals from Max Plank Society. In 2022 There was an open-access book deal allowing Plank Institute members to more easily publish books.

Things were more not always so intertwined and in 2007 the Society canceled a licensing agreement with Springer due to subscription prices and usage restrictions.