A Horseman...

The explosion rocked the area and everyone suspected that the forest fire must have caught something that contained either gas or explosives for such a thing to happen.

The noise drew all the zombies nearby to it, which was located deep in the middle of the forest fire meaning the zombies killed themselves, walking right into it.

The horde of evolved zombies paled in number to an actual horde but this didn't mean their threat was to be overlooked.

Everyone used this opportunity to slip past the defensive lines as the horsemen were nowhere to be found, they couldn't be seen as their formation was nonexistent.

Whatever caused this explosion was enough to disrupt their formation.

Or did it have to do with the distracted look on the first horseman that Rachel encountered? 

It lost interest in her a second before the explosion rocked the area, which was strange.

Rachel was lucky it happened when it did because she was about to be killed.