AMD R9 280x and 7970 users sometimes experience a graphical bug in gw2. To my knowledge this only seems to affect these two line of cards. I've experienced this bug as well with my new R9 280x. The only way to fix the problem is to alt+tab out the game and open it back up.
I ran tests to see if my card needed to be RMA. But no artifacting or any other errors came up in any other games.
To give a visual of the bug, I've attached screenshots of other users experiencing it. Hopefully someone from AMD will take a look at this and squash the bug.