Power lines came into contact with trees, sparking flames in the tinder-dry vegetation.
Under state law, utilities are required to keep vegetation a certain distance away from power lines. In three of the fires, investigators determined that was not the case, meaning the utility may have violated state law.
In a statement, PG&E says it believes its programs met the state's high standards -- adding that every overhead electric transmission and distribution line is inspected and monitored each year.
LINK: Read the full Cal Fire report
LINK: Read PG&E's full response
The report did not have any findings on the North Bay wildifres, which are also believed to have been started by PG&E equipment.