How can I find out why cmdaemon is controlling (generating) /etc/resolv.conf on a non headnode? I used cm-list-managed-files to identify that cmd is in fact managing /etc/resolv.conf for a single node but I can't find what setting, role, etc. is controlling this as no other "regular" node has /etc/resolv.conf managed this way.

In general, the /etc/resolv.conf file on a compute node is managed by the node installer during provisioning.
Please see: https://support.brightcomputing.com/manuals/8.2/admin-manual.pdf#section.A.389

The "non-headnode" we're talking about, is that a login node perhaps? Else, what function does it serve?

If you wish to prevent changes to /etc/resolv.conf, you may use the FrozenFile directive in cmd.conf.
In "/cm/local/apps/cmd/etc/cmd.conf" add the line:
FrozenFile = { "/etc/resolv.conf"}

Once the above config line is added to cmd.conf, restart the cmd service.
# service cmd restart