Or perhaps it's some kind of 'living up to the stereotype' thing, like how black people back in the pre-Civil Rights era used to adjust their mannerisms while around white people in order to fulfill their expectations and not get yelled at or (considerably) worse. In other words, these Nazis have spent so much time interacting with adversaries who speak bad German (if any) themselves that they've started to pick up their mannerisms in order to save time while bellowing threats.