Consider this flow:
- edit source code
- Cmd-Tab to SourceTree repo browser
- open repo and review changes
- Cmd-Tab back to editor
- make further edits
- Cmd-Tab back to SourceTree
At this point you are *often*, but apparently not always back in the repo browser instead of the the actual repo.
The beahviour I'd suggest would be to always tab back to the last used SourceTree window.
