It comes down to politically manufactured ethnic based hate and resentment as well as resentment over the First World War (the two countries were not very close beforehand to put it mildly). If you want to play that game you would likely find evidence that the Germans 'started it'. That being said it does not excuse the retaliatory actions of the Red Army obviously during their victorious campaign west.

/u/estherke explains in much greater detail than I could the German atrocities as they advanced through the Slavic nations and Russia proper.