Any service runs dhclient (either directly or via other service), changes /etc/resolv.conf and sets PEERDNS=no in /etc/sysconfig/network-scripts/ifcfg-<iface> to prevent dhclient from editing /etc/resolv.conf. Then it stops dhclient (directly or via other service).
Dhclient rewrites /etc/resolv.conf with backup even with PEERDNS set to "no".
Code for backing up and restoring of /etc/resolv.conf was removed from dhclient-script.
dhclient doesn't rewrite /etc/resolv.conf when stopped.