"System OOM encountered" events are shown in the "default" namespace

Issue

• In the default namespace, "System OOM encountered" events such as the following are shown:

  12:21:58  default kubelet workernode-1.example.com    SystemOOM   System OOM encountered, victim process: apache2, pid: 2408956
  12:23:32  default kubelet workernode-1.example.com    SystemOOM   System OOM encountered, victim process: nginx, pid: 212591
  

• In the kubelet log, the following "Got sys oom event" log messages are shown:

  $ journalctl --no-pager --unit kubelet | grep "sys oom"
  May 15 20:01:17 workernode-1.example.com hyperkube[4436]: I0515 20:01:17.320234    4436 oom_watcher_linux.go:76] Got sys oom event: &{1847163 ruby 2022-05-15 19:58:03.69161355 +0000 UTC m=+4595119.834387920 / / }
  [..]
  May 16 05:48:03 workernode-1.example.com hyperkube[4436]: I0516 05:48:03.244591    4436 oom_watcher_linux.go:76] Got sys oom event: &{203857 nginx 2022-05-16 05:44:48.65789055 +0000 UTC m=+4630324.800664920 / / }
  May 16 07:09:29 workernode-1.example.com hyperkube[4436]: I0516 07:09:29.839504    4436 oom_watcher_linux.go:76] Got sys oom event: &{1806564 apache2 2022-05-16 07:06:15.09893255 +0000 UTC m=+4635211.241706920 / / }
  

• What is the root cause for those messages?