It could just be that for a lot of people their only exposure to German is that clip from Der Untergang where Hitler gets super pissed and it does indeed sound like yelling in German is way more aggressive than yelling in English - but the other people in the room don't yell back at him and there's nothing aggressive about their speech.

That and the videos of people pronouncing words in different languages and of course ambulance is said in a normal tone while Krankenwagen is yelled because stereotyp funni, but that's based more on how the words look when written down I think.