Update. Under /var/log/httpd/error.log, I see a bunch of error messages like below;
[Sat Dec 19 11:52:29.570139 2015] [mpm_prefork:emerg] pid 9628Identifier removed: AH00144: couldn't grab the accept mutex
[Sat Dec 19 11:52:29.570275 2015] [mpm_prefork:emerg] pid 9630Identifier removed: AH00144: couldn't grab the accept mutex
[Sat Dec 19 11:52:29.570738 2015] [mpm_prefork:emerg] pid 9629Identifier removed: AH00144: couldn't grab the accept mutex
[Sat Dec 19 11:52:29.572450 2015] [mpm_prefork:emerg] pid 9631Identifier removed: AH00144: couldn't grab the accept mutex
[Sat Dec 19 11:52:30.147715 2015] [core:alert] [pid 9625] AH00050: Child 9628 returned a Fatal error... Apache is exiting!
[Sat Dec 19 11:53:12.328530 2015] [mpm_prefork:emerg] pid 9627Invalid argument: AH00146: couldn't release the accept mutex
and when I googled through the keyword, "couldn't grab the accepted mutex, I found below articles.
http://help.directadmin.com/item.php?id=411
When I applied "Mutex posixsem" on /etc/httpd/conf/httpd.conf, it cured the symptom.