There are people who carelessly burn something without taking proper precautions and just plain ignore conditions. There was recently someone living nearby who consistently burned garbage (which is illegal to begin with), in a wooded area, not clearing the area around the burn pile, not having a water hose on the ready, and... too often... during dry and/or windy conditions. I'm sure that kind of carelessness is how a lot of these fires get started.

Now people who intentionally start fires in order to make a giant bushfire or wildfire, how do authorities know? How do they tell the difference between that and carelessness? Unless there's a confession. And under the kind of dry conditions I've heard about, it's all too easy for fires to get started from lightning strikes. So these bushfires probably would've happened anyway.