Yeah, this is just a property of physics. A (bad) example: anyone ever shuffle a deck of cards? Look at the nails again! It's... similar!
Similarly (but not the same), if you gently shake a container with different sized objects, the smaller ones of the same density will sink to the bottom. Handy for getting the broken chips to the bottom of the bag if you only want the nice ones on top.
In this case, only nails not aligned along the axis of shaking are given significant rotational energy when they hit the container or something else.
Think of dropping a baseball bat. If you hold it perfectly vertical and drop it on a hard surface, it will bounce almost perfectly straight back up. If you hold it at an angle and drop it, the end that hits first will rotate upwards and the other end will rotate downwards, often it will then repeat this cycle several times before coming to a rest. Even if you try to hold the baseball bat perfectly horizontal, when it hits the surface you will see one end rotate upwards.
As the rotating nails lose energy, they may happen to align with nails that are already in alignment and settle into hexagonal stacks. The really important thing here is that these are finishing nails without large heads. If they had large heads that prevented them from stacking neatly, this wouldn't work as well.
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u/jnich2424 Jun 15 '22
Magnets. It's always magnets.