This happens because when two Ender pearls teleport the player at the same time, they move the player to their respective positions, causing the player to be launched.
I'm using an ender pearl with no cooldown that can be obtained with the following command:
'/give @s minecraft:ender_pearl[minecraft:use_cooldown={seconds:0.01}]'
This may be part of a larger issue, as I do not think ender pearls used to put the player in an airborne state when thrown onto the ground.