When this happens one of the members in the RAID array has likely gone out of sync with the array, this is the resyncing check that's occurring and it's 86% complete.
When the RAID array is in a healthy state the output will look similar to this:
$ sudo cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdc1 sdb1
2930266432 blocks [2/2] [UU]
unused devices: <none>
Why did this happen?
I've had these happen from time to time when one of the members gets out of sync. However if they continue to occur then it's likely a good indication that one of the members is starting to fail, and so you should heed the warning and fully diagnose all the members to make sure they're in good health and also identify which HDD (member) is failing and plan to replace it sooner rather than later.
Some distributions may perform a check periodically so you might be getting your array in this state due to these scheduled checks. I believe Debian/Ubuntu may perform a check like this weekly, look for a entry in your system's crontab files or /etc/cron.d directories.
Lastly you can manually trigger a check such as this by echoing the command "check" to the RAID's
sync_action file under
# echo check > /sys/block/md0/sync_action
If you need to pause an on going check then you can echo the command "idle" to stop it.
# echo idle > /sys/block/md0/sync_action