This is a new feature of exchange where it will upgrade the index before all of the members of the DAG are updated. Then replicate the updated index to the other members of the DAG The previous behavior was to only update the index schema after all members were upgraded.
As with most things Exchange take a look in your event log for more detailed information. You should see a large number of events
"Microsoft Exchange Information Store service will perform an internal schema upgrade on (Folder) (xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxx[x]-xxxxxxxxxx). The upgrade will bring the version from (0.143) to (0.144)."
Microsoft Exchange Information Store service has completed an internal schema upgrade on (Folder) (xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxx[x]-xxxxxxxxxx). The upgrade has brought the version from (0.143) to (0.155).
The time it takes to complete depends on the size of your environment.