Why is the ManyToManyField automatically populated with existing categories?

Question

Django

It seems that he did not indicate anything unusual, and when creating an article, existing categories are automatically put in the category field.

class Category(models.Model):
    # ...

class Page(models.Model):
    # ...
    parents = models.ManyToManyField(Category, verbose_name='Родительские категории')

How to make an independent selection of several categories?