It could be related to this post:
Basically, you had too much tender and the system still allowed you to buy something but catches up and puts you into the negative tender instead of removing what you bought.
You could also try a cache reset if you didn’t buy anything to see if it shows you the correct amount.
If you have issues with this its best to move this to the Bug Report forum so it can get eventually fixed. You can do this by mousing over the pencil in your opening post.
The same for the Renowned Explorer, sometimes randomly going on its own after zoning into a new area. Bug report!
Is this the BNET shop home page so prior entering WoW, or the ingame home page after logging in, the welcome screen with news and things? Sometimes mines is slow to load too after logging in with my toon so hm…