Many BBCodes use uppercase as can be seen here http://xenforo.com/community/help/bb-codes with some lowercase used in addition e.g. FONT=Arial. However, some BBCodes, when entered using the rich text editor buttons or other means, are in lower case. I made a test post here to show this: BBCode case testing. As you will see if you edit the post (mods) or reply to the post (everyone else) and look at it in the BBCode editor, the following BBCodes are in lower case:
[quote] (when replying to a post or using multi-quote)
I think these have been like it for a while but I really only noticed when using the [QUOTE] from the rich text editor buttons after I replied to a post and viewed it through the BBCode editor.