haven't seen any dying in my neck of the woods..{northern NSW on coast} i would think for sudden death of gum trees like yours, it would come down to 2 likely possible causes
1}root rot......if you've had drought periods, then had loads of rain with boggy soil etc, that could be a cause, tho you probably would have noticed yellowing of the leaves, and browning off before they went.
2} only other reason besides age, etc i can think of, would be poisoning by someone. but i doubt someone would be killing them off on such a large scale check and see if you can spot any drill marks in base of trunk, where they could inject poison, diesel. etc just my opinion, i'm not a qualified arborist. but have my horticulture certif's.
