I use web access daily and now when I open a document it does not open a new tab. I used to be able to have multiple tabs open.
It's been a while since I used version 9 of Web Access, but my recollection is that it would never open a document in a new browser tab. We had our own tabbing system within a single browser frame. The only exception to that was if you opened a folder under certain circumstances (that don't apply to documents), then it would open that in a new browser tab or window.
The only thing I can think of is if you were clicking on links generated by another application to open documents in Web Access. A change here would be caused by a change in that integrating application since the browser has already decided to open a new tab or not before it even contacts the Web Access server.
Version 10 of Web Access is the other way, the new UI always opens a document in a new tab.