Gw2 has crashed... or has it?
Well, it stays on top because it’s supposed to open when the game crashed or stopped responding, but there are cases where it shows up but the game keeps running however closing the error report will close the game.
Could you post what error is it? Usually the 2nd line in the details box will tell what happened. The only case I can think of that would keep the game running is audio issues, because it seems to be ran in a different thread.
Also, if it’s constant for you and nobody else seems to have the issue, it could be good for you to contact Customer Support, maybe they can help in your case.