I sometimes am reading things on the board and end up with lots of windows open. It gets sometimes catches me off guard and I post to the wrong forum, the one I was at earlier.
Can I predict when a new window will appear?
Oops…you’ve done it again 
This is an ATMB question?
As far as I can tell, any link within a post will open a new window. Any link on a forum “main” page will open in the same window.
(As for how, it’s all to do with how the link’s been coded in HTML)