Not sure why it restarted, but I got a call from Bruce that dhcpd was running again. After a bit of poking around, I did:
systemctl stop dhcpd.service
systemctl disable dhcpd.service
After this, ps -ef | grep dhcpd no longer showed a dhcpd process, and
systemctl list-unit-files | grep dhcpd shows dhcpd.service as disabled (rather than enabled)
Hopefully, we won't have to do this again...