Under Index Management, all categories were reindexed at the same time this morning. It was either someone did it, or the cron job did it.
But I confirmed that no one did it. I wonder if Magento cron job now reindex data. I know it didn't before, and I don't want any reindex in the cron job.
Here is my cron set up
*/5 * * * * /usr/local/bin/php /home/xxxx/public_html/cron.php -mdefault
*/5 * * * * /usr/local/bin/php /home/xxxx/public_html/cron.php -malways