Why is rigidbody2D calling OnCollisionEnter2D method?

I got 1 gameobject with 1 child under it.

child components:
script with OnCollisionEnter2D(Debug.Log("child"));
circleCollider2D

for parent
boxCollider2D
rigidbody2D
script with OnCollisionEnter2D(Debug.Log("parent"));


Debug Log with all of the above:
parent
child
parent
child
etc.

If i remove parent rigidbody2d:
child
child
child
etc

if i remove parent boxCollider2D:
parent
child
parent
child
etc

Why is it not just the collider calling OnCollisionEnter2D? This seems like the most...

Why is rigidbody2D calling OnCollisionEnter2D method?