Generally trees have symetric trunks that taper from the ground to the end of the branches. If a large bulge develops over time it is likely the trees growth reactions to defect within the trunk. This tree pictured in Kalorama has a significant bulge. This can develop over dot defect, ant nest or infestation of large grubs. Some times these can be associated by a distinct half disk, fruiting body of the fungus growing on the decaying timber within.
Some trees can have this managed provided it doesn’t occur on the main trunk of a large tree threatening property. These should be inspected early to try and retain if it can be done. Unfortunately most require removal as the issue is made worse with time.
This clearly shows the point of failure at the bulged region. There are large grub holes exploiting the soft rotting fibres. For a large Eucalyptus regnans like this in Olinda it was a fortunate result.