It's almost a certainty that the MDS (metadata server) was situated on a mirrored RAID (prob RAID10). I'm guessing that the RAID system itself (software MDRAID or some HW array, DDN or something like a NetApp E-Series) corrupted the data under the FS that the MDS used, which I'm also assuming was XFS.

Lustre, for those who don't know it, is a cluster meta-filesystem, with separate metadata and object servers, each sitting on top of host file systems/RAID/storage.