During delta node recovery, ns_server needs to be able to access vbucket state information during bucket initialization (i.e. before the bucket is fully warmed). This information is needed to prepare the vbuckets for delta node recovery (i.e. doing such things as maybe setting vbucket to replica state, deleting vbuckets with incompatible vbucket histories, etc.)
There's a 180s timeout to get the vbucket state information, but recently in the field we've seen a case where it can take longer than that for this information to be returned as KV doesn't return stats information on vbuckets until "vbucket maps are populated" and this can take more than 180s.