Due to climatic conditions and dry weather, fires happen in forests
There are a lot of places in the the world where forest fires are caused due to extreme heat and the easy availability of dry wood and tinder. Forest fires are quite common around the world.
The causes may be natural but the Brazlilan govt. has been very lax in taking precautions and also in controlling fires.
The fire in the Amazon rainforest is a natural calamity and not completely man-made