Why is it that big German automakers are worried about Chinese retaliation?

Because they voluntarily put themselves in the position to be retaliated against. Think Germany’s voluntary dependency on Russian gas recently. That didn’t work out very well either. But for whatever reason, this is what Germans do.

Germany launches 11th-hour bid to avert trade war with China – Germany wants the EU to set tariffs on electric vehicles at a low level to avoid severe retaliation from Beijing…

Germany’s position was “problematic,” he said: While big German automakers still entertain good ties with Beijing, that’s not necessarily the case for smaller businesses, meaning “the German economy as a whole has an interest in a more assertive policy towards China.”