I used to think the same thing... I don't think I've had a "traumatic" upbringing, so how in the world did I happen to develop BPD!?

However, if you read this article, you could begin to understand how BPD could develop out of a seemingly ordinary childhood. In particular:

"Trauma does not necessarily need to be acute (sudden/intense). It can be chronic (long-lasting) and might be relatively undramatic. This is the case with the Invalidating Environment that Marsha Linehan identified."

Also, this site provides a few clues as well:

"An invalidating environment is not the same thing as an abusive environment, although abusive relationships are certainly invalidating. Invalidation can be quite subtle and may reflect a general way of interacting."

Finally, it does also reside within a person's perception:

"It is important to remember that people tend to experience relationships and interactions differently. This means that what one person experiences as an invalidating environment is not necessarily experienced as such by another. It is possible that individual temperaments affect a person's general sensitivity to invalidation, but everyone has times when they are more vulnerable or sensitive.

It is important to note, however, that invalidation - as it relates to the development of borderline personality disorder - is not a periodic experience, but a pervasive one."

Hope this helps!