It's just a trend within the Polandball community. The first Polandball drawings depicted Polandball as upside-down, and it just caught on, and is now the accepted norm.
Polandball comics are poorly illustrated stories depicting international drama. Each country is represented as a ball (with exceptions like Israel and Nepal) and they act as temperamental as people. The stories may depict crises or relations between countries. Look at /r/polandball on Reddit, it's got some good stuff.
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