Pilot fatigue causes inattention. Don't over extend yourself just to make a buck. That extra hundred dollars isn't' worth a life or the cost of a new aircraft. Mostly we notice when it hurts. The very conditions that caused the fire to start, now keep it going.
Fires can also create their own weather pattern.
A large fire can create its own winds thereby dramatically affecting planning. California seems to have more than its share of fires. There are others, but I mention these folks because they use helicopters frequently while fighting fires. CDF is currently updating their water bucket video to help pilots train for the fire season. Once again, I just barely scratched the surface of all you need to know about fighting fires with helicopters.