Hi Brucolino,
Yes I think it was previous travel history to countries with health risks that caused the request for a chest x-ray and medical ( I spent 4 months in the Philippines in 2013 working on a dive project). A lot of people applying for this visa haven't been asked for one, I think it varies depending on history and circumstances, as Nemesis pointed out.
The health check came back clear though and according to ImmiAccount it says health clearance provided - as of the 15th November 2017, so I don't see how that could be the issue causing delay...