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Hi. That is very funny and I have to do this with my kids at home later.In my opinion it is a superposition of waves. As you can see in one video from the top the liquid moves to the wall of the bottle when falling. The center is empty. That is because of the mass inertia and the cohesion and adhesion of the liquid and the bottle. When the bottle hits the ground the liquids moves down into the cap with high speed. This centrical wave superpositions in one small point. Like a monster wave in the ocean. Waves superpose and result in a very high waterpeak. This motion is like a drop falling into water but reversed direction.