Came back from lunch to find this charming error message: HA initiated a failover action in cluster prod in datacenter example.
This is not the kind of thing you want to see on a Friday afternoon.
A quick check revealed that there was in fact no HA incident happening or waiting to happen.
The quick fix was to remove HA from the cluster and then re-enable it. However if this happens again I will definitely be raising it with VMware support.