I say it has Slavic origin. I just googled Katrina Müller (the most common German family name is Müller) for sites from Germany and got three results. I then googled Katrina Schmitz (also a very common name) and got two results. With Katharina Müller I got ca. 9.000 sites (without the doubled links, say 5-6.000).
At any rate, the words spoken by these idiot Germans was uncalled for!
Actually, "Katrina" or "Katharina" stem from ancient Greek. Some say the origin is "aikaterine" (which might be related to "hekateros" or "hekate"), others (the majority) says it comes from "katharos", meaning pure.
Anyway, the Romans took over that name from the Greek and through Latin it has spread into other languages as well.
While in English the standard form would be "Catherine" (short Kathy), the standard German form is "Katharina" (short Katrin). In some other languages the standard spelling is "Katrina", e.g. in Polish. But, as there were waves of Polish immigration to the western German Ruhr area (which can be traced through family names such as Schimanski or Kowalski), the spelling "Katrina" also exists in Germany. However, the standard German spelling is "Katharina".