(Image Credit: Medium / Rocket Lab)
The engine in question, Rocket Lab's Rutherford Engine is using a technique named regenerative cooling. It passes the fuel through the nozzle wall before it reaches the combustion chamber, cooling the nozzle in the process. The big pipe going down to the bottom of the engine is responsible for feeding the fuel into the nozzle. I could not find a whole lot of information about the inside of the engine, but it probably looks similar to this:
(Image Credit: u/christophertacon on Reddit)
There the fuel gets piped up in vertical or slightly spiraling lines. We also don't know a whole lot about the fuel injector. (If anybody finds anything useful about any part of that engine: please let me know!). The fuel probably gets injected into the engine quite soon after the cooling process.
This is where the speculation begins:
I have two theories. Both of these could be true, maybe they are even linked:
- I could imagine the streaks being the result of some sort of oscillating feedback system: A cooling channel gets hot and expands. It subsequently lets through more fuel causing it to cool again and shrinks back down. It is important to mention here, that the bit of the engine changing color is the vacuum nozzle, which is attached below the cooling system, but would still get the effects of the hot gases downstream from that assembly.
- Fuel injectors have a habit of creating visible patterns even outside the engine. In the picture below is the exhaust of a Falcon 9 second stage single(!) Merlin Engine. The Pattern is produced by the injectors being single holes rather than concentric rings: There are points in the combustion chamber where this is an imbalance between fuel and oxidizer because it is closer to an injector of that specific resource and there are points with a better mixture ration. This results in varying temperature and composition of the exhaust yielding visual patterns.
(Image Credit: Peter Thoeny on Flickr, Emre Kelly / FLORIDA TODAY)