Plumber with a theory as to what's happening here. I believe that before this video began the hot valve on the shower has been opened. You can see that when the hot on the left side of the sink is opened there is hardly any water pressure. This makes me think that the hot water is shut off and all you're seeing is the remaining water in the pipes draining out. When the cold water is turned on there is a very obvious rise in pressure. With the hot valve on the sink still opened water from the cold side will rush into the hot side, filling the hot pipe with cold water to the point where it reaches the hot valve of the shower mixer, then coming out the shower head. We call it "mixing" and it's why even if I'm only making a repair on the hot side of a water system the cold water must be shut off too. To prevent people from opening both hot and cold valves on fixture and flooding my work.