Archer %/s (KMS Unlimited and GMS Tempest)

what is happening at 18 seconds when all the mobs are being pushed back away from the wall?

Spiral Vortex fires a projectile like Iron Arrow or Pierce. When it hits a monster, it will do knockback and splash damage. The splash damage is slightly delayed, so when you hit a monster, you will do the splash a fraction of a second later. When you hit a large number of monsters, there can be a long time before all the splash damage is done. Especially in this KMST version, where Spiral Vortex's splash did 5 hits instead of 2, there is a slight delay between when each line of damage is applied.

When a monster is hit, it will be knocked back in the direction you are facing. That's why if you are facing left and use an AOE attack, monsters behind you are pushed left towards you instead of being pushed away from your character. In this case, since the splash isn't done occurring, it will still do damage as the Wind Archer turns to face left. The monsters then take damage and are pushed left because that is the direction the Wind Archer is facing.

This is still possible in the release version of KMS, but nowhere near the extent you see here, since there are only 2 hits for Spiral Vortex's splash instead of 5. So while you may still see monsters suddenly being pushed towards you if you turn around suddenly, they won't still be taking several hits of splash damage like in the video because only two hits are applied.