How long ago did the leaves turn brown? Did/do you water at the base, or did you expose the roots to look for rot? There's not much that kills them quite that fast, except frost. I have had issues with other species doing the same thing, though.
As for the oil/fire issue, that all depends on how dry they are. If well-watered they are no more explosive than any other tree. They go up like candles when they are very dry, but not in most garden situations. The ones in Australia are going through record drought and heat which makes them super flammable because they are drier than they've ever been. You do need to watch for wandering troops of Koalas, though. They can defoliate a good-sized tree in decades.
