Local conditions
Bank erosion is the mechanism that catches people out. Every high flow takes a little more material from under the trees along the water, and the loss is gradual and invisible from above. A tree that has stood on that bank for decades can be sitting on far less root anchorage than it was five years ago, with nothing in the canopy to indicate it.
Flood debris compounds it. Material carried down in a flow catches on trunks and low limbs and stays there, adding weight and increasing the surface the next flow pushes against. A tree with a raft of debris packed against it is loaded far harder than a clean one.
Away from the water, the ordinary rural picture applies. Open grown trees with wide, heavy crowns stand over fence lines, sheds and access tracks, and the region's storm season arrives on ground already softened from the wet. On a property with one way in, a failure across the track is more than an inconvenience.