Hi.The fire had burned a patch of ground about sixty feet square, moving forward until it was almost opposite the place where Barbie and Sea Dogs were facing one another. And there it spread - west to the edge of the highway, east into some small dairy farmer's four acres of grazeland - not raggedly, not the way grassfires normally advance, with the fire a bit ahead in one place and falling a little behind somewhere else - but as if on a straightedge.
This is Under the Dome by Stephen King. Can you explain the underlined sentence, please? Especially "a bit ahead" and "falling a little behind" parts.
Context: At the place where the plane had crashed into the Dome, Barbie and Sea Dogs met accidentally and were watching what was happening.
Thanks in advance.