According to the news item, what make(s)the major bush fire spread?

(28)The authorities in the Australian state of New South Wales have urged people living in the Blue Mountains to leave their homes as strengthening winds and high temperatures threatened to spread major bush fire.(29)Forecasters warned that Wednesday could be the worst day since the crisis began more than a week ago. Exhausted firefighters have been preparing for their most challenging day yet: they have been carrying out controlled burning of key areas. The ideal was to deprive fires of fuel and prevent them travelling. On Monday, they joined up two large fires to make one that is easier to manage.(30)Around 60 fires are burning in New South Wales, for there are 17 of them are still uncontrolled. Almost 120, 000 hectares have been ruined by the fires so far and more than 2000 firefighters have been trying to get them under control.