Power lines need to be separated from anything that can potentially conduct electricity to earth. If the minimum distance is reduced by an item which conducts electricity, then the high voltage electricity can ‘jump’ from the wires to the conductor and then travel through that conductor, to earth.
Electricity companies will ensure that easements are kept clear of trees and other items which can potentially reduce the space between the wires and earth in order to maintain a minimum safety buffer. Unfortunately, heavy smoke can allow electricity to conduct temporarily which can cause a bolt of electricity to jump from the wires to ground, potentially starting a fire.
Whilst power companies will argue that there is no record of that occurring during bushfires, no one can tell whether spot fires in the vicinity of high voltage wires are caused by electricity arcing or flying embers, so just because there is no record, doesn’t mean it doesn’t happen. The phenomenon has been proven scientifically, so it’s not a question of whether it’s possible, but a question of how likely and what the potential impact would be.
Power companies will also argue that their ‘systems’ can detect a fault in milliseconds and cut power to a fault. Whilst this is true in theory, in practice it’s a totally different scenario. There are numerous ongoing worldwide studies conducted by the power transmission industry into high voltage circuit breaker failures in an attempt to improve their reliability and better predict failures. Relying on a breaker to accurately detect and respond to a fault assumes ideal conditions, not real world conditions.