I've seen this problem reported a couple of different times. Also, I know quite a few users that have this issue. What happens is when I have a Sublime Text window open with a few tabs, I change focus to another window while I'm doing some other task. When I switch back to Sublime, I click on the tab that I'm wanting to view with the same click that is giving Sublime Text focus. Quite often, this results in the tab being pulled out of the existing window and being opened up in a new Sublime window.
I'm pretty much able to reproduce it every time I want. My current setup is a Sublime window with two panels. About 5 tabs in each. I click on another window to bring focus to it (I'm on a 3 monitor setup). Then, when I click on the tab in Sublime that I want to view, I click and then immediately move my mouse down into the content of the window. If the click and the move motion happens close enough together, the problem occurs. I've been VERY careful to make sure that the mouseup event happens before the actual mousemove event but it still happens. The problem seems to happen a little more when the computer is laggy.
This problem is REALLY annoying and I know quite a few people that would love to have this fixed. I hope this helps. Below is a little more information about my system in case it helps.
Sublime Text 3 - build 3047
Windows 7 x64
8 Gigs ram
Two nvidia graphics cards with 3 monitors
Aero is disabled