Admin area – categories falling out of tree

1. I’m using categories, with one level of sub categories.

2. When I assign a sub-category to a post, the category (I’m still in the admin system here) goes to the top of the category tree (it no longer appears to be a sub category)

3. When I view source, as well as pulling it out at the top of the

• , it also has a class of ‘popular’ (I’m in ‘all categories’ tab, not ‘most used’ here)

4. If I look at any other Posts, the category is where it should be – safely nestled in the tree in it’s rightful place (so it doesn’t seem to care if it’s ‘popular’ at this point)

I have the following plugins installed;

* Custom Post Type Template Redirect (active)
* Custom Post Type UI (inactive)
* NextGEN Gallery (active)
* Verve Meta Boxes (active)

…although i’ve played around with these though, and not sure they would affect the admin categories box.

Any ideas how to get categories to stay in their tree structure?