If I remember correctly that was a result how the fallout was transported via the jetstream - and if it did rain, hence a rather non-uniform distribution. The first fallout cloud went from Ukraine over Poland to Scandinavia but it did not rain down. A second cloud went westwards over then Czechoslovakia and then southern Germany, hence the impact. The German Agency for Radation Protection has this map of Caesium ground contamination in 1986:

https://www.bfs.de/SharedDocs/Bilder/BfS/DE/ion/notfallschut...

The mushroom thing is because of bioaccumulation: Mushrooms seem to ingest the particles from its surrounding ground/ground water, hence a higher concentration of radioactive material in a smaller volume. And then wild boars eat those mushrooms, concentrating it even further. Caesium 137 has a rather short half life of only 30 years, but through the process of accumulation/concentration still today meat from wild boars shot in that region gets tested and is often over the allowable limit to eat.

In Bavaria testing of venison is mandatory and consumers have the right to see the measurement protocol for every piece of sold meat.

Because the contamination varies greatly, depending on where it rained during a short timespan in 1986, the amount of usable meat also varies, but is usually between 50% and 70%. The rest, which is not safe to eat is bought by the state.[1]

People are always quick to call Germans crazy because of their attitude towards nuclear energy, but Chernobyl had real world implications to our daily lives and to a degree still has to this day.

[1] https://www.jagd-bayern.de/jagd-wild-wald/jagdpraxis/rcm-mes...