It's not so much the bark, but that it's FULL of eucalyptus oil that is *extremely flammable*!I'm in Palo Alto quite a bit for work and the first time I saw them I was floored at how messy they were. The shreds of bark that fall off have to be an instant accelerant to any fires in the area.
The oil isn't all bad, though - it's also a very good natural pest deterrent for the trees and other things around it.
But, one errant spark and POOF!