I guess with modern cities everyone thinks of the modern parts of the city first, but some cities (including London) have old town centers or other older parts (or slums, they can work in a similar way for this scenario).
If your fire starts in that part, you have more "options".
Houses are still made of inflammable material and might not be particularly well kept
This is not the "pretty old center" tourists look at, this is just some residential area where the buildings have been around for a long time and people just kept them up without doing major renovations or just doing cheap renovations. (Yes, that is borrowed from the Grenfel fire. It can happen everywhere.)
Streets are narrow and houses close to each other
Old streets were not built for large fire trucks. While the fire brigade struggles to get through, the fire is jumping fast from one dry attic to the next. In a city that normally does not cope with forest fires, the availability of means to extinguish a fire from the air might be small and would have to be brought from far away, which gives the fire even more time to spread.
Make it social
If it was not an accident but an angry mob setting fire to a given "enemy", this same mob would try to keep anyone away who tries to extinguish it. At some point, they would have to leave the dangerous area, but at that time the fire can already be out of control. The mob has also added fuel to the fire in the meantime or hid explosives or other dangerous material which makes it too dangerous to get close or use standard methods to extinguish the fire.
Let it start in multiple locations at once
Even the largest fire brigade can be spread out thin until help arrives.
Let it smolder
Maybe it is something wrong with the gas supply which leads to smoldering, undetected fires in multiple locations. Or electric transformer stations have been serviced by a company who made multiple mistakes that also lead to smoldering which at some point breaks into a fire. This scenario also somewhat "helps" with the fire breaking out at multiple locations at once.
A smoldering fire can already be spread far before it is breaking out or detected. And situations in which there is smoldering are often harder to extinguish as well.
Of course, not all of this works when "someone accidentally dropped a candle". But from the beginning, this is a hard start, as that someone obviously sees the candle fall and can act immediately.