Stiff upper body. That little arm flail in the first vid when you skidded - I'm guessing it happens a lot on a more challenging terrain or in worse conditions. That's because your arms are glued to your sides and your spine isn't relaxed enough. When you are riding icy, bumpy steep terrain you need to be fluid through your waist to compensate for slips and skids with the body rather than flapping arms. As for the butters you're fighting the board a bit.
And what do you carry in a backpack this big on what's essentially a bunny run on a tiny hill? Dead body?