To answer briefly, it came from the National Socialist ideology itself, but there's also some other factors that must be taken into account.

19th Century Reunification and Volkische ideal

Our story begins with the rise of national identity after the French Revolution and the fall of the ancien regime ; socioeconomic factors which formed a rather strong what we'd today call 'middle-class' and increase in urban, educated population. Alongside this came the concept of nationalism, and consolidation of power into the people identifying themselves as a certain nation.

To not dwell on the political aspects of the German unification, in terms of art history, this came parallel to the rise of the Neoclassicist style. As a retort to the flamboyant Baroque and Roccoco styles, which were seen as lavish displays of power of the aforementioned ancien regime (nobility and clergy), Neoclassicism in its theoretical sense and its alignment to nationalism movements was meant to push two agendas - first, one of purity and ellegance, and second, one of continuity of culture - the use of classical greek and roman architectural elements for example, symbolizing a clear cultural link to the ancient Greece. To put it in layman's terms, it was meant to show that Neoclassicism is a pinnacle of 2000 years of european culture, not some kind of a revolutionary new style out of nowhere.

Notable examples of these are Friedrich Schinkel's Altes Museum in Berlin ( even though Schinkel died in 1841., this building is seen as a pinnacle of development of his architectural style), and theory wise we have Gottfried Semper and his treatises on the decline of historical styles. Semper argued that each time needs its own style, instead of copying the historical ones, and anticipated the development of Art Noveau and Modernism.

Rise of Modernism, interbellum period, Weimar Republic and Bauhaus

Through the rise of Art Noveau, Art Deco and Modernism after the turn de siecle, The Great War happened, we know Germany lost the war and found itself in a socioeconomic crisis - after the revolutions of the 1919. and the establishment of the Weimar Republic, Germany went through a time that established the cultural legacy we know of today - seeking to break apart from the Kaiserreich cultural legacy, carried on the wings of new, modernist theories and seeking its paradigm in free, uninhibited cultural development that will usher Germany into a new, free, democratic age. The pinnacle of this was the formation of the Bauhaus school in Weimar, later moved to Dessau. Bauhaus was imagined by its founder, Walter Gropius as a totaldesign thinktank, where intellectuals would devise ways of fixing the world through architecture and industrial design, avoiding and preventing the pitfalls of the old world that brought us in the chaotic situation of the post-war 20s, as accentuated by the Dada movement.

Other non-Bauhaus initiatives of the time include Ernst May, an urban planner and architect that in 1925. made great plans for the development of Frankfurt am Main, from the general urban plan to the design of individual workers' apartments.

Rise of NSDAP, 1933. and Entartete Kunst

The overall cultural development of Weimar in the 1919-1933 period was seen by the Nazis as one of the most dangerous threats to their ideology of National Socialism. Modernism was dangerous because it was its clear anti-thesis, favouring liberalism, upsetting and questioning the status quo, breaking away from tradition, and to a degree something for the 'ivory tower intellectuals', and not the Volk. Given that a number of the intellectuals and modernist artists of the time were either jewish (f.ex. Marc Chagall, Erich Mendelsohn) or outright socialist (Hannes Meyer, second director of Bauhaus), anything related to modernism was seen as not just dangerous, but an integral part of the 'jewish-bolshevism conspiracy' against the german Volk (common term for it was 'Cultural Bolshevism')

In 1933., after the NSDAP's rise to power, parallel to the formation of Goebbels' Ministry of Propaganda, a subsidiary state body, the Chamber for Culture with its subchambers for art, architecture, music, film, etc, was formed, with the clear goal of censorship - a new, Nazi Germany needed culture that falls in line with its politics - one that glorifies the Volk, its 'proud' history and militarism. Here, the logical step was to take the step back into glorious days of German reunification, before the 'shame' and 'betrayal' of the Great War, so Neoclassicism, seen as 'pure from jewish influence' in architecture, alongside Socrealism in painting was reintroduced as the cultural dogma in line with NSDAP's policies. For examples of this, we have a number of examples, from Nazi propaganda posters (socrealism) to Albert Speer's Germania designs (neoclassicism).

It all started by banning jewish and modernist authors from state owned galleries, museums and universities (Bauhaus was closed down completely in 1933.) and culminated with the infamous exhibition 'Entartete Kunst' - 'degenerate art' in Munich 1937. (which then toured Germany and post-1938, Austria) , showcasing cultural production of modernism, like painting, sculpture and music ( Jazz music specifically ) as something chaotic, evil and abhorrent to the German national spirit. The organizers of the exhibition went through extra trouble to hang the paintings skewed and add snide remarks to them, in order to underline their 'degeneracy'

To conclude, the Nazi's hatred of modern art and the '180 turn' ,as you called it, comes from their own ideology, as a reaction to the cultural bloom of the Weimar Republic, theories about an 'international jewish conspiracy', and the understanding that a country's cultural production, or rather, control of it, is an important aspect of the nazi state's interior and propaganda.