I think referrals happen in cases where there are queries on results. There is an FAQ for panel doctors here:
http://www.immi.gov.au/gateways/panel_doctors/conducting_medicals/faq/medical/
Its not something to immediately panic about, sometimes they just want to check out any meds you take, or slightly high blood pressure etc. I think the basic idea is you get a grade a or b. a no significant issues or grade b, issues. If something serious was found, your doctor would have been in touch before diac got the results.
This is my understanding anyway, it can add to processing time.