I think it is mostly learned while we grow up. Either it is a copied behavior, meaning a parent models the behavior, or a reaction to a very aggressive parent.
It can be learned later on though. I have very little passive aggressiveness in me, I think it is a horrible communication style. However, I know I am somewhat passive aggressive, or I’d say less confrontational, with my husband’s family, and even my husband in certain things. It’s a result of “you get what you give.” After years dealing with them I basically adapted to their style when dealing with them, because they simply can’t handle real communication. Luckily, this is far and few between with my husband, but it does happen. I have to say that becoming more passive aggressive with them is a result if me simply not caring as much. It’s where my mind had to go to stifle. I can’t help but wonder if people who tend to be very passive aggressive are less attached to people. I’m not assuming they are, but it feels that way to me.
My downfall personally is I’m a little too aggressive, too much temper, raise my voice a little too easily. I’m better about that with my husband than with my family, but I still could work on it more. I still think it’s the better if the two though. At least while yelling we are still figuring out some sort of understanding and resolution, but it’s not a great way to go about it. In fact, when dealing with someone who is passive aggressive once you raise your voice the whole thing gets worse. It’s unbelievably frustrating.