Hypothetically, you find out your dad changed his last name because he hated his father. Let’s say he changed it to the last name of a fictional character he really liked, then went completely no-contact with his family decades ago, before you were even born. Then, 20-something years later, you discover your grandmother is still alive, and your last name isn’t actually your dad’s birth last name.
What would you do?
I think they mean that their father went no-contact with his own parents/family (OPs grandparents and extended family) prior to OPs birth.
Phrasing aside, it’s still asking strangers to speculate on the reasoning of one person. That is definitionally a subjective answer. Even if someone magically gleams the reasoning, it cannot be confirmed without their dad confirming it.