As always man is the cause for many problems, including wildfires, in fact 90% of them. Campfires left unattended are a big issue, another is arsonist or pyromaniacs.
Even Mother Nature is harsh, if a large grass plain undergoes a dry season there is a 10% chance of wildfire starting.
Lightning is the last of these causes, this falls under the 10% of natural disasters.
Great Fire of 1910: The Great Fire of 1910, also occasionally referred to as the "Big Burn," is believed to be the largest single fire in recorded U.S. history. It burned more than 3 million acres in Idaho, Montana and Washington — in all, a total area roughly the size of Connecticut. There were 87 fatalities from the fire and 78 of those were firefighters.
Oakland Firestorm of 1991: One of the worst urban blazes in modern history, the Oakland Firestorm of 1991 began as a relatively small grass fire in the Berkeley Hills. But after being persistently fanned by the strong seasonal "Diablo winds," that brush fire eventually grew to consume 1,520 acres, including more than 3,500 homes, apartments and condominiums. Such destruction — and in such a densely populated area — meant the disaster carried one of the heftiest price tags for wildfire damage in U.S. history: an estimated $1.5 billion.
Yellowstone's Summer of Fire: The summer of 1988 saw the largest wildfire breakout in the recorded history of Yellowstone National Park. By the time the fire subsided, more than 2 million acres had been scorched — roughly a third of the entire park. Miraculously, no lives were lost as a direct result of the flames, even though as many as 25,000 firefighters had been dispatched to battle the conflagration.