After rebooting, check whether Asterisk is running as root (it should be running as asterisk).
If it’s root, there is probably a startup file left over from when you installed Asterisk (before installing FreePBX).
You can probably disable this via
chkconfig asterisk off
Now reboot and you’ll probably find that Asterisk is not running at all. If so, you need to issue
fwconsole start
at boot time to (among other things) start Asterisk properly.
A simple way is to add the command at the end of /etc/rc.local
There may be quirks in your system that are incompatible with this method. See How To Load Asterisk 13 as asterisk instead of root on CentOS 6.7 & FreePBX 13 for more details.