Ron snapped. Before Malfoy knew what was happening, Ron was on top of him, wrestling him to the ground. Neville hesitated, then clambered over the back of his seat to help.
"Come on, Harry!" Hermione screamed, leaping onto her seat to watch as Harry sped straight at Snape –– she didn't even notice Malfoy and Ron rolling around under her seat, or the scuffles and yelps coming from the whirl of fists that was Neville, Crabbe, and Goyle.
(Harry Potter and the Sorcerer's Stone)
I’ve read several times this book, yet it’s not easy to understand the bold-faced part whenever I read it. The antecedent, the whirl of fists, doesn’t seem to be equal with Neville, Crabbe, and Goyle. Is this a unique expression of the writer or common in English?