If you are referring to the fires in Dartford/London then the 'major incident declared' simply means that they can call on resources from further afield to assist or to provide cover for the areas normally covered by the units attending the fire.
It is more of a procedural thing.
>Think someone threw a fag out the window?
Most likely - the heath is a busy area for dog walkers etc so one of them might have dropped a cigarette.
Dartford Heath usually has at least one fire like this each year. The heath is crisscrossed with roads and firebreaks so the fires are generally contained in a smallish area of just a few acres.
There was/is a second fire in the area @Joyce Green that has burned an area of grassy marsh and destroyed five houses.
With two significant fires in close proximity the local crews call on units further away, hence the declaration of a 'major incident'.
Anon is fairly local & knows area well.