what is this ?
UFO sighting in Southern California.
SAN DIEGO UFO 12-22-17
It's a rocket launch. As the rocket ascents through the atmosphere, the atmospheric pressure rapidly drops, causing the exhaust plume to fan out in a fashion that may appear unnatural. The sun is already over the horizon for people on the ground, but this rocket is many kilometres up and therefore catches the sunlight, causing it to light up in the sky.
It looks like the clip caught the moment just after the separation of the first stage and the rest of the rocket. The two smaller, denser stripes are probably two sets of motors used for different purposes, but which use similar technology. The rightmost denser stripe is likely caused by the retrorockets, which fire after the first stage has stopped its engines and after the two stages have disconnected, to physically push the first stage and the rest of the rocket apart. This allows the rest of the rocket to safely continue on its path, without interference from the first stage. Because SpaceX recovers most first stages, it also makes sure that doesn't get blasted by the second stage firing its engines, although the first stage was actually not recovered intentionally in this case. It may have been a practice run to gather data or how they made sure the first stage ended splashing down in a safe area.
The second denser plume, which is attached to the big plume, are probably the ullage motors firing. These are helper rocket engines which push the rocket just a tiny bit, so that the fuel in the tanks settles on the bottom, allowing for it to be pumped out properly rather than the fuel just floating around in the tank. Another explanation might be that the denser plumes are unignited fuel gasses being pumped out of the motors in preparation of firing them, while the turbopumps are spooling up. You don't want to ignite rocket engines before the mixture and engine cooling is appropriate and sufficient. The trail could also be a combination of ullage motors and pre-ignition exhaust, as it seems to be widening.
You can even see the first stage in the plume, turning itself around by firing its control thruster engines repeatedly to orient itself for re-entry.