The basis for my very non-scientific observation is that I am a native Dutch speaker, who's conversational in German and has a Polish partner. German fits quite well in between Polish and Dutch in terms of features: Dutch has few, Polish has nearly all, and German has quite a few more than Dutch ;) Similarities between Dutch and German are more easily understood since they've a recent ancestor, but for Polish and German we must go back much further. Yet, my untrained eyes see a sort of continuity that seems to cross the language-family barrier, which could make sense because of that shared geography of German and Polish. I know that Poles have a history of fervent conversationalism that favors (grammatic) complexity, so perhaps it's a hobby that spilled over to German-speakers. Or vice versa (I know less of German cultural history).