There have been small local outbreaks in Germany since lock down was eased, most relating to abattoirs, churches, care homes or large family gatherings. Most are only in the tens of cases, some a hundred or so, this is the first on this scale because the owners clearly took their eye off the ball and didn't spot it quickly enough.
This is the whole point of a good track and trace system, local outbreaks are inevitable but you can stop them spreading to the general population.