Once the fire is contained within a fireline, only part of the work is done. To prevent the fire from restarting later, crews now need to comb the burned area in search of hotspots--burning stumps, logs and other debris that could threaten containment. Every single one.
One would think the danger is over now. But that's where the danger lies: believing the danger is over.
Exhaustion plays a role. The firefighter may have spent 16 hours battling the fire, and after a short break, it's time to go back to work. With the rush of adrenaline gone, it's easy to fall into the belief that you can let your guard down. But you can't. Falling trees have been known to kill people during the mop up stage. You also have to watch out for burning stumps, with hotbeds of coals that are hidden by ashes. If you step into one, you could end up with serious burns. Rocks are loosened when there is no vegetation to hold them in place, and they can tumble at any time.
And then there is the risk that the fire restarts. In a heartbeat you go from mopping up to active fire suppression.
Here's a video to give you an idea of what mop up entails. This lucky crew was able to have water available. Quite often this is not the case. When there is no water, you are stuck with using dirt to smother the flames, removing oxygen, or by chopping the burning roots or logs apart so they cool down.