Some theories are clearly more accurate than others. And it's also manifestly clear that people who cite arson, or human errors due to drunken parties, as a possible source, are doing so to distract attention from a different and far more plausible explanation.
As a matter of accuracy, I should acknowledge that no-one can be certain that climate change is the ultimate cause of this or indeed any other extreme weather event. But it's far and away the most likely driving factor. Australia had suffered a prolonged drought, has experienced several record-breaking temperatures, and the weather in turn is driven by other freak conditions. Such conditions are only going to get more frequent in a climate change scenario. In those circumstances, when action can be taken that is highly likely to at least reduce that threat, why wait until we reach the impossible standard of certainty before trying to do something about it?