I don't know how it is supposed to work, but ntp dhcpcd hook changes /etc/ntpd.conf by erasing its content. And openntpd stops working.
This hook adds ntp servers to ntpd.conf if DHCP server sends them and removes them when interface is down. Other content shouldn't be touched and this apparently works for me exactly with openntpd, so I don't know how to reproduce this issue. BTW, you can disable this hook in the /etc/dhcpcd.conf (add 'nohook ntp.conf' there).