I see by your profile you are in Australia, here in america depersonalization is not always caused by trauma. it can be caused by many different things. example when i went out with my wife last night we were in a crowded restaurant, I got over stimulated by the crowd and started feeling my depersonalization symptoms...numb disconnected, spaced out feeling.

for some people its stress, for others anxiety, for others its PTSD, depression any many other things too.

my suggestion talk with your or a treatment provider in your location, they will be able to explain it according to whats what where you are.