Auto detached rows when deleting model?

I am using a pivot table implement many to many relation on Criteria model.

I've noticed that when deleting a criteria, the related rows for the deleted criteria in the pivot table is automatically deleted too.

Although I didn't add any setup for that, is this normal? Can someone explain this please?

Below are the models:

class Criteria extends Model {

    protected $table = 'criterias';

    public function criterias()
    {
        return $this->belongsToMany('App\Criteria', 'criteria_criteria', 'central_criteria_id', 'criteria_id');
    }


}

class CriteriaCriteria extends Model {

    protected $table = 'criteria_criteria';

}

via Chebli Mohamed