Can confirm, and can elaborate. This seems to happen if the player goes down at least 2 meters consecutively at all, including sneaking down stairs, or jumping from 1 block downward while sneaking (which causes you to fall 2 blocks, as you jump upwards slightly more than 1 block). It still is caused even if the fall happened out of range of the skulk sensor, and then the player moved in range while sneaking. It is not caused by jumping in place, or moving around long distances horizontally without falling.
Aurora_Spirit
Can confirm, and can elaborate. This seems to happen if the player goes down at least 2 meters consecutively at all, including sneaking down stairs, or jumping from 1 block downward while sneaking (which causes you to fall 2 blocks, as you jump upwards slightly more than 1 block). It still is caused even if the fall happened out of range of the skulk sensor, and then the player moved in range while sneaking. It is not caused by jumping in place, or moving around long distances horizontally without falling.