"Pucey feints and tricks Johnson, goes through Bell—wait, where's Flint going? It looks like he's chewing Iroko out—no, is he giving instructions?—Pucey almost scores! But Wood grabbed the Quaffle, now Spinner has it—"

And Victor's broom gives a sudden, frightening lurch, completely out of his control.

- flying is not like flying anymore.

It happens again. It's as though the broom's trying to buck him off, even though as far as he knows Nimbus Two Thousands do not suddenly decide to buck their riders off. 

...

It is not reasonable to try to stay in the air under these conditions. He descends. Carefully.

He wishes. It's gone completely out of his control, zigzagging this way and that every now and then making violent swishing movements that almost unseat him. The game continues heedless, "Slytherin in possession—Flint with the Quaffle—passes Spinnet—passes Bell—hit hard in the face by a Bludger, hope it broke his nose—only joking, Professor—Slytherins score—oh no..." The Slytherins cheer their score, and it appears that no one's noticed Victor's broom's slow ascent.

Oh, somebody's noticed it all right.

Muninn takes off and circles, looking for anyone in the crowd who is, say, staring fixedly at Victor's broom.

Well there is this faculty member.

Also this one! Who unlike the other one is moving his lips silently.

And suddenly people notice, and start pointing at Victor. His broom had starts to roll over and over, trying to jerk him off it.

Uh-huh.

He lands on Quirrell's turban.

He startles. "Get off, you stupid bird!" he says, waving his hands at Muninn.

Snape is startled, too, as are the people around them.

And Victor's broom stops moving on its own just as Fred and George reach him to help.

"Victor! Are you alright?"

He takes off again. Mission accomplished.

"Something was -"

He spots Muninn leaving Professor Quirrell's head.

"- happening to my broom," he says. "Now it isn't but I'm afraid it'll start again."

"Should we get Hooch to stop the match?"

Up Dayo appears. "What happened?"

"His broom went crazy, he thinks it'll happen again."

"I think someone was cursing it," he says. "I don't know what to do."

"Who? Why?"

"D'you think you could hold on if it happens again? 'Cause then we'd know it wasn't a fluke or something and be able to tell Hooch..."

"...I can hold on at least a little while."

"We'll hang around. And Flint's giving Dayo the evil eye."

"—yeah, I'd better go, the game's sorta resumed—" Off he goes.

All right. Back to the game, then.