There's a better, longer video on Reddit. At the beginning of the video, it sounds like the Ram Air Turbine (RAT) is deployed, which would suggest a dual engine failure.
Guessing it's either a foreign object ingested into both engines, probably a bird strike, or fuel starvation.
And here's a YouTube video showing a Ram Air Turbine (RAT) being deployed on a 787 for comparison: https://www.youtube.com/watch?v=oFBCGf50Trc&t=69s
It could be the blurry object under the fuselage, just to the right of the left engine.
New videos came out and so far the prevailing theory is that flaps were retracted too soon. Most likely instead of gear up.