Recovering primary JUNOS image

When I booted my SRX device I saw the following output on the console;

 WARNING: THIS DEVICE HAS BOOTED FROM THE BACKUP JUNOS IMAGE

  It is possible that the primary copy of JUNOS failed to boot up
 properly, and so this device has booted from the backup copy.
 Please re-install JUNOS to recover the primary copy in case
  it has been corrupted.

This had not happened before. According to junos documentation I can either install a new junos image to be on the safe side or copy the backup partition to the primary. As you can see my current partition is backup. Once I snapshot the backup to active partition and then
reboot after a while, I could boot from the primary one.

root@srx> show system storage partitions  
Boot Media: internal (da0)
Active Partition: da0s2a
Backup Partition: da0s1a
Currently booted from: backup (da0s1a)

Partitions information:
  Partition  Size   Mountpoint
  s1a        292M   /
 s2a        293M   altroot
  s3e        24M    /config
  s3f        342M   /var
  s4a        30M    recovery

root@srx> request system snapshot slice alternate
Formatting alternate root (/dev/da0s2a)...
Copying '/dev/da0s1a' to '/dev/da0s2a' .. (this may take a few minutes)
The following filesystems were archived: /