So graphs like that should be read together with information about how the bush management changed over time. See this long thread for example: https://twitter.com/JediJeremy/status/1208997157346242561

I believe in recent past we've gone through: "everything's burning, let's put it out", "we've had massive fires, let's do planned burns", and now "let's defund organisations that can do effective bush management". This will explain some of the changes visible. The current area is due to the buildup nobody managed - neither by government organised burns, nor fire stick farming.