Most of the German expelled from Central and Eastern Europe into Germany (primarily West Germany) were from areas that had been ethnically German long before the Second World War or even the twentieth century. In fact, a portion of the postwar Polish state had been "East Prussia" (a province of Germany before the war), which, as a result of the postwar settlement after the First World War, had been divided from the rest of Germany by the so-called "Polish Corridor." This section of land was given to newly reformed Poland after the First World War so that the new state could have access to the Baltic Sea, but this "corridor" divided Germans and ostensibly stood as the pretext for Hitler's initial invasion of Poland in 1939.

Similarly, the area known as the Sudetenland in Czechoslovakia had been historically German, and it stood as the pretext for the dissolution of the Czechoslovak state by Hitler and Chamberlain at the Munich Conference of 1938. In the immediate aftermath of the war, Poles, Czechoslovaks, and other ethnic groups whose states contained large numbers of Germans (in the millions in Poland and Czechoslovakia) forcibly expelled these populations so that the German nation could never use the populations as a pretext for conquest again (which is not to say that the Poles and Czechoslovaks were not swayed by the emotion of having just lived through the war). The massive number of German refugees that the United States had to deal with between 1945-1950 became a point of contention between the American government and the Czechoslovak government, because these were indeed refugees without homes in Germany whom the United States and (what would become) West Germany had to accommodate. Whether or not these expulsions were justified is another discussion.

I can tell from the angle of Germans (Volksdeutsche) living in area in Serbia where I live, is that they were settled by Austrian emperors throughout 18th and 19th century, in newly liberated areas from Turks as they had knowledge in many crafts and were known as skillful craftsmen, unlike domestic Serbo/Hungarian population. Craftsmen were bulk, but of course there was number of officials, officers, lower nobility, basically Austria was trying to assimilate those areas into empire.

Their purpose as well was to, when called, serve in army, usually against Turkish incursions. Indeed people (not only Germans but Hungarians, Serbians, Croatians, Romanians) who were living in special territorial sections (military frontier) of Austrian empire and who were given land or help with their businesses were expected to serve in military in an event of war.

Now, with the fall of Austria in 1918. this area became part of a Yugoslavia and Germans were hostile towards it, especially after Hitler rose to power. German population rushed to join Kulturbund, which was alliance of German organisations in YU. So, when Vermacht marched in 1941, basically Germans were organized and prepared to take over rule. In my town, for example, native Germans commited crimes with the help of Vermacht against Serbians. 7th SS division ''Prinz Eugen'' was formed out of German population in Serbia. All this, plus wish of new communist authorities after the war to get their hands on German properties, capital and factories is an answer why Germans had to leave.