The Met left those commemorating Sarah alone all day until about 7pm, and then again today (as there is a constant stream today too)
So what was different about the time from about 7pm onwards? It wasn't that there was a gathering, that had started at about 5:30. It wasn't the silence and applause, or the size of the crowd at that time.
Perhaps it was the increasing numbers? (people were still streaming on to the Common at 7ish and that the crowd had become so densely packed that it was impassable) and that there was a lot of chanting?(loud speaking/singing being an extra covid risk).
It wasn't a vigil that was dispersed, it was a protest