Node drain hangs forever

During a node upgrade, kubectl drain keeps retrying with "Cannot evict pod as it would violate the pod's disruption budget." The Deployment has 2 replicas. What is the likely problem?

apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
  name: api
spec:
  minAvailable: 2
  selector:
    matchLabels: { app: api }