I'll cut right to the chase: Brandishes are far too powerful for what they are. Hence, to make them a bit unique (and less of a keep-away charge attack weapon), have the final explosion in a brandish-line charge attack have a high knockdown chance but push the enemy towards the player. This means the monsters caught in the entire explosions will be brought close to the player if they don't keep moving, so you cannot reliably use the sword for crowd control. Instead, the enemies are pulled back, so you can smack them with a sword again. The final explosion has a high knockdown chance so enemies won't just hit you in the face every time you use the charge attack, however in a full party on elite brandish charging will be more dangerous.
This also makes Voltedge and Glacius a better choice for "control" opposed to having full control over crowds with a combuster while dealing much more damage.
The final explosion does not deal any more or less damage, only higher knockdown.
This would make the Combuster, Acheron, Obsidian edge go down in popularity but would make the Voltedge and Glacius go up since their statuses would resist the new mechanic.
Why not trade in the Brandish line's knockback for a higher Knockdown rate (Like Nitronome VS DBB), this would still allow the users to stay safe but would be more team friendly.