I've taken a look at the bug:
1) I have Word open on monitor #2 with tabs.
2) I'm using a program on monitor #1 (Firefox or Xplorer(2), for example).
3) The active program on monitor #1 opens some activity that takes focus from the parent app to a child window (save file dialog, or a delete file dialog...).
4) The child window/process ends, and when focus returns to the parent app, it's moved to monitor #2.
This behavior doesn't happen every time, but enough to be pretty annoying
.
If I close the app that is using tabs, then this behavior stops.