GR translation: rolling right behind

[David Morris]

First was the sharp crack, followed by the heavy explosion, which was
“rolling right behind” it (in sequence).

On Sun, Apr 24, 2022 at 1:55 AM Mike Jing <gravitys.rainbow.cn at gmail.com>
wrote:

> 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?
> --
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>