Just to make clear: I also think it's unlikely, but it is a long-stop possibility.
But anyway, I had a dig around. It looks like QF is reducing its HKG-SYD frequency from the current 14 down to about 11 or 12 for a few months between April and July, but coming back up to 14 in the middle of the northern summer. This may be a decision it's just made, which would seem to fit. So some of the FCO-HKG flights will no longer connect directly with a day flight from HKG-SYD, eg Tue ex-FCO. If that's what's caused the problem, there's a decent case for QF re-routing the pax via LHR to meet the day flight ex-LHR (which is ops QF, FWIW).