Hiya
Good question..
The DSM-IV-TR placed PTSD under the anxiety disorders umbrella due to the typical behavioural manifestation..
however; because the root cause of PTSD is trauma of some variety, such as abuse.. I would think that it fits better in the abuse section of this site..
I'm unsure of the site owners motivations for placing it here but i would think their rationale was the same as mine..
I've had a quick search to check the latest version of the DSM because i thought i heard talk of PTSD being reclassified in there as well. In the latest edition, the DSM5.. PTSD is now placed very well in the ''trauma and stress or related disorders''.
A much better fit as far as i'm concerned.