But other major fires whose names have entered the record books were tied directly to actions by people, including the Klamathon fire (reckless burning of debris ), the Carr fire (a wheel rim scraping pavement), and the Mendocino Complex fire (hammering a metal stake into a wasp’s nest).

Despite all of the warnings and precautions, human error, it seems, always finds a way.

“Some of these things are really quite unbelievable when you hear about them,” said William C . Stewart, a forestry specialist at the University of California, Berkeley. “But they just occur with a certain probability. They just do.”

Mr. Stewart has seen several fires that began when a lawn mower scraped against a rock on a hot day, sending sparks into the dry grass. The Sandalwood fire was the first time he had heard of a garbage truck’s load igniting a wildfire, but not much surprises him anymore.

“There’s just an endless series of things that people do to create sparks and fires,” Mr. Stewart said. “This time of year, when everything is bone dry, it really is just like kindling.”

Shawn Melvin was idling at an intersection in Calimesa with his son on Thursday when the smoking trash truck sped by. Mr. Melvin, 36, a diesel mechanic, has lived in the area his whole life and had never seen anything like that, so he followed the truck and had his 8-year-old son record the rising smoke.