At what time point in the video do you see the protesters break through the police line? I scanned through the 1.5 hour video and didn't find it. It seems like the whole video was filmed from the same location.
It's hard to tell but it looks like a door was either breached or opened to the far left. There is a loud thud happening at the same time but it's hard to tell if that's what caused it.
It's seems to be off the side of the barricades so it doesn't appear anyone was "let in".