Ender Dragon causes kinetic damage.

Getting hit by the Ender Dragon with extreme knockback into a block can cause kinetic damage even when elytra aren't engaged.

In this particular instance, the Ender Dragon was in its phase where it lands on the exit portal. While under the Ender Dragon, its hitbox collided with and pushed the player with extreme knockback. However, when the player hit the edge of the portal, it caused kinetic damage.