So what caused this?
A video making the rounds is from someone who filmed it while he encountered flames from wildfires in the Porter Ranch area, which is a suburb of L.A. He starts out by filming FLAMES and smoke coming from manholes in a grocery store parking lot.
According to the text in the video, embers make their way into the sewers, and that sets debris and vegetation in the sewer system on fire.
He also shot video from inside his car as he passes through the wildfires and into the clear.