Fortunately I have enough family around here that my mother (living in another country) never comes up in conversation.
I did run into an old family friend recently, and she seemed shocked that my mother and I are estranged. I smile politely and tell her really, I’m fine, I don’t feel the need to reconnect with her.
As I told my sister (who still has a relationship with our mother) I don’t hate her. I’d have to care to hate her.
Well, sweetie, if you’re such a disappointment to her, why bother her anymore with your presence? Since you’re such a problem. Really. You’d be doing her a favor removing such a painful irritant from her life.
Go tend your flower garden and leave the weeds alone. 
My sister is 4 years older than me (I’m 21) and I haven’t seen or talked to her except momentarily in about 3 years. My sister turned into a major druggie and dropped off her daughter at the father’s house and never came back. The sad part is my sister lives maybe twenty minutes away from me but she doesn’t want to get off the drugs so I rarely see or talk to her. It really does not affect me much maybe because I lost all respect for her for treating her child that way.
If someone asks I tend to be more forthcoming that those who have responded so far. I speak positively about plans with my immediate family, but also say something about how relations with my birth family are strained and we don’t see each other. I drop it at that, but surprisingly often this opens a window for someone else to share a painful story of estrangement. There is healing and comfort and affirmation in these discussions. Look at the healing words spoken here because of Adoptamom’s willingness to be vulnerable.
I tell them, “Yes, I’m spending the Holdays with my family.” My wife and my kids are my family now, and I wouldn’t have it any other way.
Sorry for the delay in responding; I’d overlooked your post earlier.
Well as I said in the OP nobody knows why. Sounds strange to me too. I also mentioned later that the mom is a narcissist. Now I’m not an expert but I understand that narcissists are not always there for their kids and they take any type of perceived criticism real hard. So it’s possible that some variation on the following dialogue has taken place - maybe even more than once:
Estranged Cousin: Mom, I need to talk to you about a few things that happened in my childhood, like …
Naricssistic Mom (interrupting): WILL YOU GROW UP ALREADY!!! I’M TIRED OF HEARING ALL THIS CRAP FROM TEN (OR TWENTY OR THIRTY) YEARS AGO. I WAS A GOOD MOTHER!! A GREAT MOTHER!! WHAT I DID FOR YOU!! AND DO YOU THINK RAISING YOU WAS A DAY AT THE BEACH?!..
Reading all the above stories has been very helpful to me in getting some clarity about what might have happened. I don’t think I could have even come up with the above ideas a week ago.