This is speculation, but apparently there were protesters on both the civilian entrance side as well as the promenade (where you saw the protesters scaling walls). When the protesters breached the promenade entrance, I assume there was some sort of stand down orders for the police at the civilian entrance, presumably so that the officers wouldn't get pinched.

Hence the weird dual screen of protesters breaking through windows/protesters staying within the cordons. They weren't part of the same cohort.

This was also my reaction. I think many viewed it that way, like it was an expected part of them being there