"Forest fires are common in the Amazon during the dry season, which runs from July to October. They can be caused by naturally occurring events..."

"... such as by lightning strikes, but also by farmers and loggers clearing land for crops or grazing. Activists say the anti-environment rhetoric of Brazilian President Jair Bolsonaro has encouraged such tree-clearing activities. In response, Mr Bolsonaro, a long-time climate sceptic, accused non-governmental organisations of starting the fires themselves to damage his government's image."

From "The Amazon in Brazil is on fire - how bad is it?" (BBC).