The Hughes Fire near Lake Castaic erupted at around 10:30 AM (PST) (18:30 UTC). As shown by infrared surface temperature imagery, a very large area of brush suddenly started registering very high temperatures (black dots) within 30 minutes.
https://cameras.alertcalifornia.org/?pos=34.5501_-118.7402_12&id=Axis-WhitakerRidge1
Oh, this is near Interstate-5, the only north-south artery for the LA basin aside from 101 near the coast.
My money is on burn scar mudslides. There are many of them now.