Sheer ferocity and force, these fires have gained so much momentum, quite a few of the areas have provided great fuel, (tinder dry fuel), accompanied by those hot, dry, very strong winds...remember this was EXTRAORDINARY weather conditions, it was stuff that seasoned, experienced firefighters had never seen.
Radiant heat in these situations is the big killer, some of the fire just gained so much force, so much speed, it was like fireballs.
http://www.cfa.vic.gov.au/documents/radiantheat_brochure.pdf
Sadly, some of the people shown on tv were running around in shorts, bare skin, bare feet exposed, they need to be educated you must cover up, (see CFA link)...fire plans need to be drawn up, fire is unpredictable, some got caught with the sheer ferocity and speed, and also the change that hit, the different weather conditions, wind altered direction, they were caught in this, cars and drivers do become disorentiated in smoke, radiant heat, (remember it was extraordinary conditions) just took everything in it's path, incinerating everything in it's path), leaping valleys, fuelled by wind and fuel...there is no guarantee anyone would be safe huddled in their car, such the ferocity of the heat.
Fire crews and units got caught,
experienced firefighters...kept remarking how ferocious the fire had become, the weather conditions...there were so many variants, warnings had been issued for days and days, people tried to get the message out of the exceptionally horrid conditions, but it still took people by surprise. (see Bill's original texting from CFA).
If you listen to the firefighters, the people involved, you hear the sheer enormity of what they were up against.