Blue jacket guy was beating on white trouser guy (Who wasn't the stabber, just trying to subdue him), police officer thought the situation was under control now that they'd all arrived and subdued blue jacket guy. In doing so feeing up subduer and stabber. Stabber then stabs police officer.

It was a clusterfuck either way but blue jacket guy might somehow get the blame for this as he was subduing the man holding down the stabber.