Editing & Post-Production
Compression artefacts
Also called: artifacts, macroblocking, blockiness
When an encoder has fewer bits than the picture needs, it throws away detail, and what you see is the shape of that decision: blocky squares in dark areas, banding across a smooth sky, and a general mush whenever the frame moves fast.
Confetti, rain, water, foliage and fast pans are where it shows up first, because everything in frame changes at once. Heavy film grain added in post does the same thing, and can look far worse after YouTube re-encodes it than it did on your timeline.