Dark crystals

The story that they created was that the demons were using these monster waves to disrupt the human kingdoms.

It wasn't just our kingdom that would be experiencing this increase in monster activity during this time, it was all of the human kingdoms that would be experiencing this.

That worked for the player since the increase in mobs would make it easier for them to gain experience and items.

As for how they were doing it…

Dark crystals.

That was how.

These were not normal crystals, but rather special crystals that increased aggression in monsters through dark magic. So even those monsters that would normally be docile would become aggressive after being exposed to the dark magic of these crystals.