If you have enabled the cluster autoscaler, it may have marked some nodes as unschedulable to ensure efficient resource utilization. This is normal behavior and should not cause any issues.
Check if any taints have been applied to the nodes. Taints can make nodes unschedulable unless a pod has a matching toleration
kubectl describe node <node_name>
Nodes can be marked as unschedulable if they were cordoned manually using the kubectl cordon command.
kubectl uncordon <node_name>