The RocketLab YouTube video STP-27RD Launch - 05/05/2019 shows the exhaust plume from the second stage engine evolve as the atmospheric pressure drops. During this time there are frequent outbursts of presumably hot glowing particles in the exhaust, some of them appear to be moving far slower than the exhaust velocity.
The engine burns RP-1 and LOX. Usually when we talk about soot for example, it appears to be a continuum of so many fine soot particles (I'm guessing microns in size or smaller), but these have got to be much larger in order to be so individually bright and distinct.
What are these exactly, and why are the so prevalent from the Rutherford engine?