The writer uses a because they want to say it could be any woman, and if it's a person's mother, they don't know whose mother, since they meant "anyone" when they wrote "you".
Compare it to:
When it's cold out, you should put on warm clothes: a sweater, a coat, or furry underpants, perhaps.
Note that the actual rules that describe when we use a vs the vs other choices to introduce a noun are very complex, although grammars often present them in oversimplified ways.
And, since your thought about the topic is leading you to confusion about the grammar, Yes, it's possible, in the view of most psychologists, that one injury could lead to a psychological defense like this.
The concept of defense is not explained very well, here, but I think it's meant to give a simple idea of the word's meaning, not a lesson on the topic.
But to scorn someone is to express strong dislike toward them, or lack of caring. If a mother told or showed a child that they dislike them, it seems easy to imagine that it could be very serious and traumatic.
It's probably easier to imagine something like a serious violent act or sexual abuse, etc., that migh lead to such a defense.
If you research defense mechanism, I think you could learn more about what it means.