Fires can occur from both natural and unnatural sources. Unnatural fires would be fires that occur due to human intervention. Natural sources of fire can produce wildfires. They include lightning strikes, volcanic activity, sparking from rocks falling onto other rocks, and spontaneous combustion. Spontaneous combustion is where a naturally occurring chemical reaction is exothermic and produces heat in a confined area surrounded by combustible material. Bacterial breakdown of organic waste in a landfill is an example.
Unnatural sources of fire are caused by human intervention. The question of whether human activity is natural is probably best left to a separate philosophical question.
Natural can, and generally does, mean anything that exists and/or occurs in nature. Trees, oceans, continents, thunderstorms, animals, flora, wind currents, etc. are all natural occurrences. Fire can be the result of quite a few different natural events. Wildfires, specifically, can be caused by lightning strikes, volcanic eruptions (which naturally create fire by themselves), falling rocks (such as a meteorite), and even spontaneous combustion. The likelihood of a natural fire depends on many factors like aridness, temperature, fuel availability (brush or other dried plant material), and a way to ignite the fire – or a source.