Too many variables to track, in my opinion it's exactly as @smoon described above - Google is placing automatic blocks on IP ranges that hit certain limits or thresholds, so its going to depend on the range your server is in and which range the "offenders" are in.
It's unfortunately (or fortunately, however you look at it) likely nothing you are personally doing.
Either somebody is seriously abusing Hetzner resources to perform attacks against Google or Google is seeing degradation in their network from the amount of traffic they receive from Hetzner and finally had to do something about it.