GR translation: rolling right behind

[Mike Jing]

V25.27-33, P26.8-14   It was Friday evening, last September, just off work,
heading for the Bond Street Underground station, his mind on the weekend
ahead and his two Wrens, that Norma and that Marjorie, whom he must each
keep from learning about the other, just as he was reaching to pick his
nose, suddenly in the sky, miles behind his back and up the river
mementomori a sharp crack and a heavy explosion, rolling right behind,
almost like a clap of thunder. But not quite.

What does "rolling right behind" modify here?