Best described by a step by step:
1) Open 4 text terminals, arrange them in a 2x2 grid
2) Open firefox, make the window large enough to (partially) cover all 4 terms
3) Alt-tab to the terminal window group, all 4 are raised, last focussed one
   gets the focus
4) Close the focussed terminal
5) The focus now reverts to firefox and firefox gets raised over the
   3 remaining terminals

I realize this is discussable, but I personally find this sub-optimal, since the entire window group was selected in the alt tab, and all were raised, I would expect the focus to go to one of the other windows in the group, and the window stacking order to not change.