Full Moon

In Chapter 6 we learn that Full Moon has powerful effect on Monsters, according to Centorea it is "A night that calls out their instincts with the utmost force." Thus making their passions and behavior more extreme and reducing their self-control.

In practical terms, combined with the possibility of Kurusu being able to marry this caused the girls loose their control and use their strength in full, to the point that he feared for his life.

Amusingly enough, apparently the way to reduce the effects is to literally cool their heads.