Orange County Fire Authority officials said Caltrans crews were using heavy mowing machinery to cut dry vegetation around an access road when the mower struck something hard and caused sparks.
The surrounding brush was so dry, that it ignited and flames grew quickly.
The fire erupted along State Route 241 near Santiago Canyon Road in Irvine shortly before noon Monday. The blaze charred 180 acres and was 65 percent contained as of midday Tuesday.
Three abandoned structures were destroyed in the fire, which prompted the evacuation of the Irvine Lake campground.