SourceTree creates temporary files in $TMPDIR that are not removed when a repository window is closed, or the app exits. This happens under normal usage, apparently during synchronisation.
That directory ought to be cleaned-up when the files are no longer necessary, as under normal operation the OS only does it at a restart (or possibly a re-login). Which is not a frequent event on a workstation class computer.
Can those rare cases apply across application restarts?