Before the 19190628 The Treaty of Versailles 01, the Northern part of Poland was German territory, after the Treaty of Versailles it was Polish territory but still full of Germans, farmer, shop keepers, labourers etc etc.
The polish bolsheviks wanted them out, even though they had been living there for generations, this territory used to be called East & West Prussia.
The means used by the polish bolsheviks to remove the Germans from this territory was not pleasant, in fact it was brutal and murderous. It was the same in the Sudetenland. Hence Hitler's reaction.