Why? For no reason at all! https://en.wikipedia.org/wiki/Bloody_Sunday_(1939) >>After German Selbstschutz snipers fired on retreating Polish troops, there was a Polish reaction against the German minority and then the retaliatory execution of Polish hostages by the Wehrmacht and Selbstschutz, after the fall of the city. All these events resulted in the deaths of both German and Polish civilians. The Polish Institute of National Remembrance found and confirmed 254 Lutheran victims, assumed to be German civilians, and 86 Catholic victims, assumed to be Polish civilians, as well as 20 Polish soldiers. Approximately 600–800 Polish hostages were shot in a mass execution in the aftermath of the fall of the city as a "revenge". After the Germans took over the city, they killed 1,200–3,000 Polish civilians in retaliation, as part of Operation Tannenberg. The event and place of execution became known as the Valley of Death. The murdered included the president of Bydgoszcz, Leon Barciszewski. Fifty Polish prisoners of war from Bydgoszcz were later accused by Nazi Sondergericht Bromberg summary courts for taking part in "Bloody Sunday" and shot. https://en.wikipedia.org/wiki/Bloody_Sunday_(1939)#Background >>Bydgoszcz (Bromberg) was part of the Polish–Lithuanian Commonwealth until 1772, when it was annexed by the Kingdom of Prussia during the First Partition of Poland. As a part of Prussia, the city was affected by the unification of Germany in 1871 and became part of the German Empire. It would remain a part of the German Empire until the end of World War I. In February 1920, the Treaty of Versailles awarded the city and the surrounding region to the Second Polish Republic (the administrative region of Pomeranian Voivodeship). This resulted in a number of ethnic Germans leaving the region for Germany. Over the interwar period, the German population decreased even further.