How to: Why are some of my pods in an Unknown state?
In Kubernetes 1.6, a significant change was made to how the master treats nodes whose state cannot be determined. Pods from the node are put into an `Unknown` state and the master will not attempt to reschedule them.

In Kubernetes 1.5 and earlier, if the node has not sent a heartbeat to the master in 5 minutes, the node is deleted from the masters and the pods are rescheduled automatically.

You can force delete pods with
```
kubectl delete pods
--grace-period=0 --force
```
https://kubernetes.io/docs/tasks/run-application/force-delete-stateful-set-pod/#force-deletion
A longer discussion of the reasoning and the change is here:
https://github.com/kubernetes/community/blob/master/contributors/design-proposals/pod-safety.md