It's mostly just the fear of immigrants in general and populist right wing stuff like "they take out jobs", "they will replace us" and such. It's general xenophobia against all kinds of people, be they eastern European, from the Balkans or Mediterranean.

Ironically, studies have shown time and time again that the areas with the highest concentration of AfD-voters are also the ones with the least amount of immigration. Adding to that, the AfD has a very large social media presence and there are a ton of people spreading fear, propaganda and, to a large part, Russian disinformation. They always make the most outrageous claims and offer the most simple solutions that have no basis in reality. Some people are unhappy with rising prices and stagnating economy and are blaming it on the "new" government that hasn't had a chance to clean up after 16 years of conservative Christian rule and was elected in the middle of multiple global crises.