The two firefighters were treated for minor facial and knee injuries at Mission Hospital in Mission Viejo.
The blaze was reported at about 11:30 a.m. in a remote area about a dozen miles northeast of San Juan Capistrano, close to the Cleveland National Forest.
"Temperature is about 80 degrees, our relative humidity was about 14 to 15 so it's very dry," said Orange County Fire Authority Capt. Greg McKeown Monday afternoon.
With dry brush and low humidity, the flames moved quickly up steep terrain near Ortega Highway, a major connector between Orange and Riverside counties.
"If you took a piece of paper in your hand and light the bottom, it's going to burn much faster up then it would if it was laying flat. So the fire hits that hillside and burns with high intensity, a lot of ferocity and burns right up that hill," said McKeown.
About 300 firefighters are battling the blaze. Due to the rugged terrain, the flames were being attacked mostly from the air with four helicopters and two fixed-wing planes, but they had to stop flying at sundown.
Battalion Chief Kris Concepcion of the Orange County Fire Authority said a "disking" operation for clearing brush or breaking ground for agricultural use knocked down a utility pole, and the power lines appeared to spark the blaze. Disking usually involves a tractor pulling a device.
There was little wind during the day, and Concepcion said humidity was expected to rise to 30 percent overnight. Officials said ground crews were making good progress.