[CentOS] RAID Resynch...??

So still coming up to speed with mdadm and I notice this morning one of my
servers acting sluggish...so when I looked at the mdadm raid device I see
this:

 mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Mon Sep 27 22:47:44 2010
     Raid Level : raid10
     Array Size : 976759808 (931.51 GiB 1000.20 GB)
  Used Dev Size : 976759808 (931.51 GiB 1000.20 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Sun Nov 14 11:33:51 2010
          State : clean, resyncing
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

         Layout : far=2
     Chunk Size : 512K

 Rebuild Status : 57% complete

           UUID : f045370a:5be687e9:73e57992:06ea59e5
         Events : 0.8

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1


If done some googling but I am stumped as to what would have kicked this
off, anyone have any insight that would be great...The system has been up
for ~34 days and it kicked off at 4am local time...Here are some of my
system details...

Centos 5.5 - 2.6.18-194.11.4.el5

fstab:
LABEL=/                 /                       ext3    defaults        1 1
LABEL=/boot             /boot                   ext3    defaults        1 2
/dev/md0                /home                   ext4
noatime,defaults         0 2
/dev/md0                /mnt/data-raid          ext4
noatime,defaults         0 2
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0
LABEL=SWAP-sda2         swap                    swap    defaults        0 0

Thanks in advance....
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