One of our a/c flew straight through Steve's track that night within 3nm of the crash site 20 mins prior with only minor turb, I'd flown along the VFR lane about 1.5hours earlier and experienced moderate turb. The "Swan hill" run had flown through there without any major reports either.
The forecast was there and while no one else experienced any of the wx associated, there was obviously reason for it. Conjecture is that they may have flown into a tornado/sheer funnel and having seen them in the area I wouldn't be surprised.