Here are some answers that I have found while reading up on this...
- Dry conditions - Australia has a lot of dry areas. Most of the Eucalyptus tree species have adapted to these dry conditions and some of them can grow in almost desert like conditions. Just this alone is a receipt for fires, trees and very dry conditions.
- Peeling bark - Eucalyptus trees have smooth bark that peels off in long strips and falls to the ground. This gives any fire an abundance of material to burn on the ground as it spreads.
- Flammable leaves - Eucalyptus leaves are very flammable adding to the material on the ground to keep the fire going.
- Dependent on fires - Some species of Eucalyptus can form thickets and actually need fires to clear out the competition.
- Flammable oils - ome Eucalyptus trees such as the "Blue Gum" produce flammable oils that make the trees either burn with incredible intensity or even explode. The Blue Gum has been called "Gasoline trees" for this reason.
Of the many eucalyptus species that evolved with fire, none is more incendiary than blue gum. "Gasoline trees," firefighters call them. Fire doesn't kill blue gums. Rather, they depend on fire to open their seedpods and clear out the competition. And they promote fire with their prolific combustible oil, copious litter, and long shreds of hanging bark designed to carry flames to the crowns. Blue gum eucalyptus doesn't just burn, it explodes, sending firebrands and seeds shooting hundreds of feet in all directions. Living next to one of these trees is like living next to a fireworks factory staffed by chain-smokers.