LOS ANGELES — In the summer of 1965, Johnny Cash was living in the wilderness of Southern California when — possibly high on drugs — he sparked a wildfire with his overheated truck that blazed through more than 500 acres and threatened the lives of endangered condors.

When asked by a judge if he started the fire, he said, “my truck did, and it’s dead, so you can’t question it.” (Mr. Cash ended up settling the case for $82,000, or about a half a million in today’s dollars.)

California is experiencing its worst fire season in memory, with one million acres burned so far this year, more than twice the amount in the same period last year — and with each new blaze Californians are asking themselves: How did this happen?

The answer, in Mr. Cash’s time and even more so now, is that destructive wildfires nearly always begin with a human being, either intentionally or by mistake. There are endless ways people start fires — a discarded cigarette, a child playing with matches, a campfire, fireworks, a car accident.