getting pppd "must be root" after setting whole fs with chmod 777

I have a process called network_manager which at some point runs a command /usr/sbin/pppd. everything works fine. But then, some files becomes unreadable on the file system and I use sudo chmod -R 777 rootfs in order to get access right. Once I run this command, the network_manager can no longer run /usr/sbin/pppd, I get an error : /usr/sbin/pppd: must be root to run /usr/sbin/pppd, since it is not setuid-root I dont get it, running chmod 777 isn't changing the owner of pppd so why it's no longer working ? it worked by setting chown root /usr/sbin/pppd chmod u+s /usr/sbin/pppd but I don't understand why