Forest fires are usually caused by dry lightning or lightning not accompanied by rain. They often occur in isolated areas, and this is the reason why wildfires caused by lightning burn more areas than fires caused by human activities. Similarly, volcanic eruptions can also ignite forest fires, as the hot lava or magma burns everything that comes in its way.
Read more at Buzzle: http://www.buzzle.com/articles/what-causes-forest-fires.html