Index volumes which are marked as failed are retried when the index server is restarted, and periodically. The periodically part was added sometime ago in EV 2007, so in later versions, and all versions of EV 8 upwards the retry happens. By default this retry interval is 6 hours.
You can control it with the server side registry key :
HKEY_LOCAL_MACHINE SOFTWARE KVS Enterprise Vault Indexing
Specifies the frequency in hours with which the Indexing service should check for and reset any index volumes that are marked as failed. The default is 6.
Set this registry value to 0 to stop the Indexing service from trying to reset the index volumes.