We were once in a national forest campground in New Mexico over the Memorial holiday. On the last day most everyone cleared out. I took a walk around the campground and saw a small fire starting at a campsite. There were still burning logs in the firepit and it appeared sparks jumped out and caught the pine needles. I rushed to the campground host and he called it in on 911. We went back and started packing up. We were ready to leave when the fire trucks arrived and the fire had spread significantly over the pine forest floor. They evacuated the few remaining campers. I followed the fire on the news afterward and it had consumed many, many acres and the campground was closed down the rest of the summer.
Cigarettes are most likely to be blamed for some fires, especially along the roadsides but many major fires are started by careless campers.
When you leave your campsite, pour a bucket of water on your fire remains. If you can't put your hand in the ashes then your fire is not out. This is a common message given by the rangers.