Ok, just noticed some odd behavior...
Scenario 1: When I go to an interface category from the category listing page, then try to sort, I get no changes.
Scenario 2: When I'm viewing an interface piece and I click the category name in the breadcrumb links above the file description, the sorting magically works again.
The URLs are not the same!! 
Scenario 1 (Does not work)
Code:
http://www.eq2interface.com/downloads/cat100.html?cid=100&dp=0&sh=full&so=asc&sb=filename
Scenario 2 (Works)
Code:
http://www.eq2interface.com/downloads/index.php?cid=100&dp=0&sh=full&so=asc&sb=filename