Agreeing with the above; I got curious about the same thing once and had a talk with my psych about it. I thought of it (dissociating) as re-experiencing from a distance, and she said that actually no - the "from a distance" part is what makes it avoidance.
One of those "its really hard to remember that clinical language is clinical" things, because it never feels like "avoiding" anything, does it? But avoidance (clinically) does not require deep conscious choice - one does not always choose in avoidant behavior.
Like walking around a brick wall instead of straight into it; you wouldn't really think of it as "choosing" to avoid walking into the brick wall, you'd think of it as the normal thing to do. And yet, you did avoid the brick wall.
Avoidant behaviors are generally not considered wise or healthy (like not walking into a brick wall), but in the PTSD mind there are many brick walls we see that other people do not. Some that we don't even question, and some so large that it's a long walk around them. I think of disassociation as the long walk.