It has to do with German politics in the 19th and 20th century. Actually we could go back a couple millennia as well and talk about German attacks on The Roman Republic, the Roman Empire, the Slavs, and more. But let's stick to more recent history.

At the beginning of the 19th century, Germany was still the Holy Roman Empire, a loose confederation of more or less independent German states, largely dominated by Austria. Napoleon disbanded the Empire, annexed portions of it, and generally mucked about with internal German politics to keep the French as Hegemon of Germany. This wasn't appreciated by Prussia or Austria, the leading German states. And both participated in the Sixth Coalition that finally overthrew Napoleon. And both gained significant territories from the war.

At that point Germany was a collection of minor states, a great power in Austria, and a middle power in Prussia. For the next sixty years the dominant question for Germans was unification. Should it happen, and if so what form should it take. This turned into a struggle between Austria and Prussia for who should dominate a potential unified Germany. The Prussian ideal was a mostly Protestant Germany without the great southern German Catholic state Austria. The Austrian ideal was a greater Germany controlled by them. The whole thing came to a head in 1866 when Prussia and Austria went to war. Prussia won the war and annexed some of the Austrian allies in Germany (most notably Hannover). At that point Prussia had more than half of the future German state, and had largely excluded Austria from Pan-German entities. Four years later Prussia went to war with France, resulting in the other German states flocking to Prussia's banner, the declaration of the German Empire, and the annexation of some French territory.

In 60 years Prussia had attacked France (annexing territory), Denmark (annexing territory), Austria and other German states (annexing territory), and France again (annexing territory). By the 1870s Prussia had quite the reputation as an aggressively expansionistic nation. That reputation quickly transferred to Germany. They got to the colonization game late. But they quickly embroiled themselves in minor conflicts across Africa, Asia, and the Pacific. Nothing the other powers hadn't done, but Germany was viewed as an upstart. And it did nothing to dispel the idea that they were aggressive and expansionistic.

When WWI broke out Germany was again seen as the aggressor. While Germany nominally joined to defend their ally Austria-Hungary against Russia, they started by trying to secure their western border, invading France and neutral Belgium. That was see as excessive to their stated goal. Leaked German war aims showing plans to annex further territories in both the East and West, further cemented the idea of Germany as aggressors in the minds of many. WWII again reinforced the idea. Germany expanded in every direction invading Austria, Czechoslovakia, Poland, Denmark, Norway, The Netherlands, Belgum, Luxembourg, France, Yugoslavia, Greece (mainly to help the Italians who were already invading Greece), and the Soviet Union.

By the end of the war, the existing reputation had been firmly cemented in the minds of most. Germans were an aggressive, expansionistic people. An idea that brought back to mind the agressiveness of the German barbarians against the Romans. The Prussians used war to create an empire in the middle of Europe. That empire/state then conducted the two most destructive wars in history in an attempt to further expand.

Polandball's Reichtangle is a joke based on that historically expansionistic reputation.