It was a stray firework. I know because I read it in the Guardian.
And of course no-one could have foreseen the fans gathering in large groups, except all of the people who did.
People are blaming Cummings. And to an extent they're right. Liverpool fans are taking their cues from the top, but not from the Tories. Years of apologetics for their behaviour from the club itself and the arse-kissing media make them think that their actions are just good old-fashioned hi-jinks. When the bus incident happened, it was literally only fans of other clubs who were outraged by it, with the media just sweeping it under the carpet, and then acting all shocked when that guy was almost killed in the next round with the media failing to make any link between the violence against City and a bunch of Italians coming to the game tooled up. And here we have it again. It's a 'stray firework' among the festivities even though it clearly looks aimed and is followed by the cheers of other fans in the area. Those lovable scamps.