To me, this video shows a collection of "holes in the cheese";
1: Approach and retardant drop with a tail wind.
2: Possible loss of density altitude flying into a mass of heated air.
3: Possible erroneous readout of altitude gain due to altitude density change.
4: Possible drop in power output due to change in oxygen content while flying into smoke.
5: Possible temporary IIMC flying into smoke (can't confirm this from the video, not sure if they were in the smoke or on the other side of it)
6: Slight rise in terrain as evidenced by some images of the debris field.
7: Possible fatigue due to workload and intensity of the work.
8: Working in a timezone 8 hours different then your native timezone (not sure how long they had been in Australia prior to the accident.}
9: Pressure to continue working despite the possible fatigue and difficult conditions.
So the scenario becomes this: Flying with a strong tail wind, immediately after the drop, the pilot is immediately into what is effectively IMC, probably adding power, but suffering a power loss due to the depleted oxygen levels of the smoke column. His altimeter shows a false climb as the altitude density falls, and his wings loose lift at the same time, but he has no visual clues to see this. And all of this occurs in about 10 seconds. Add in a slight grade increase of the terrain at the same time that he cannot see, probably compounded by fatigue challenges and pressure to continue, and there are more than enough holes to add up to an ever increasing set of additive factors.
At the same time, the bravery of these dedicated individuals is nothing less than super heroic. Just a very very sad event in a tragic period of Australian history.
Respect to the crew, and condolences to those they left behind.