LOL - I'd say it started to occur a few hundred years before the boxer breed became recognised as a breed

The boxer is considered to be a German breed largely because it was developed by German breeders. But it was developed
from Belgian and French dogs. For example, the primary recognised ancestor of the boxer is the Brabant bullenbeiser. But Brabant isn't actually in Germany

It is an area of Belgium.
Italy is not so far away either, and they have some very nice dogs there. It's no great surprise to see mixing of bloodlines - it has always been the case to some degree. Personally, I doubt it has a thing to do with tail docking or heart certification. The requirements in Germany are high, and they're not changed by place of birth (meaning if a dog doesn't pass the breed inspectors, it doesn't pass and won't be bred in Germany - doesn't matter where it came from).