Find out why a Kubernetes cluster was restarted on Google Cloud

This has happened twice in the last two months - during a weekend the entire GKE cluster is restarted.

It happens even though the cluster is set to manual upgrade (release channel is set to "None").

Is there a way to find out why the GKE cluster was restarted?

As stated in documentation:

No channel

Regardless of whether your cluster is enrolled in a release channel or not, cluster control planes are always upgraded on a regular basis.

. . .

Automatic upgrades are typically performed in stages over multiple weeks. Infrastructure security is high priority for GKE, and as such control planes are upgraded on a regular basis, and cannot be disabled. However, you can apply maintenance windows and exclusions to temporarily suspend upgrades for control planes and nodes.

Use gcloud container operations list --filter="TYPE:UPGRADE_MASTER" to see recent control plane upgrades:

$ gcloud container operations list --filter="TYPE:UPGRADE_MASTER"
NAME                              TYPE            LOCATION    TARGET  STATUS_MESSAGE  STATUS  START_TIME                      END_TIME
operation-1631397153397-fc924191  UPGRADE_MASTER  us-west1-b  prod                    DONE    2021-09-11T22:05:53.397098204Z  2021-09-11T22:10:04.559303783Z