I think the honest answer on String Jumping is that short of being able to talk to a deer or read it's mind, we just don't know. There are SO many possible reasons and extrapolations not to mention for every deer that jumps like crazy there will be one that just takes it.
We know that our arrows are not faster than the speed of sound, nor is their flight silent. The deer could be reacting only to the sound of the bow, they could be reacting only to the sound of the arrow, the sound of the bow could put them on alert then they react to the arrow. Are lightweight faster arrows better because they get to the target sooner or are they worse because they make the bow louder and are louder in flight? Likewise with heavy arrows, is it better to be quieter or faster? What the heavy builds have in their favor is a better chance of penetration if something unexpected (like bone) gets in the way due to another step or a string jump. There's a comment under the video where a guy says his heavy crossbow bolts sound just like a bird in flight and he's never had one jump the string, who knows.
Same goes for the head up or down in relation to string jump. I think they're definitely onto something with the ability to use their head as a pendulum and whip their body around faster. There's also the question of which senses are getting the most attention, possibly sight while up and hearing while down, which means they could be more or less likely to react as quickly to the sound your bow or arrow make.
Likewise we don't know the deer's experience being hunted or pressured and same goes for what time of the season you are hunting. There's a lot that goes into this we'll just never know that can only be truly overcome with a rifle.