On that Channel 4 footage, the guy is shown loitering around on the fringes of a riot, and the later shot shows the actual pushing.
The police were advancing down the street, he was crossing their path, taking it slow. Now I'm sure I read he was quite drunk at the time, which would explain why he was walking so slowly.
But the police weren't to know that. They were in the middle of a public order situation (riot) and all the time they are trying to push protestors back. This guy walks in front of their path, without any urgency and could be mistaken for trying to provoke the police by walking slowly, getting in their way. That's how someone could see it.
Bear in mind, the police were on high alert, they'd been involved in skirmishes all day, and no were jumpy.
Also, as far as I'm concerned, if you are in the area of a riot, and you don't want to be involved, then walk away from it. This footage shows he was loitering around the area before the incident.
If I was involved in a riot, as a protestor or whatever, and it started to get ugly, I'd walk away. Because sooner or later the police are going to retaliate (as is their duty to restore order and quite simply to show people who's boss...they don't have time to differentiate between who's been giving it some all day and who's just protesting). Because they need to be seen to giving a bit of authority back to show people that they're not to be reckoned with, which is the whole backbone of restoring order in a riot.
As far as I'm concerned, the guy shouldn't have been there at the time if he didn't want to risk physical reaction from the police, it was a riot. The police are slowly getting their powers eroded away, powers which we need to keep order.