Maybe you had a power outage or the server crashed? btrfs is currently not the most stable file system because it’s very new and may can contain bugs. You want to have a look at: Manpage/btrfs-check - btrfs Wiki
Raid does not help you if the filesystem is corrupted in most cases because the raid controller just writes the corrupted blocks onto the second hdd. Remember: raid is not a backup.
Btw: Is it Software or Hardware Raid?