http://stackoverflow.com/questions/3559278/nerdtree-does-not-open-in-the-directory-from-which-macvim-was-opened/4831709#4831709

NERDTree buffer quit with :q

    function! NERDTreeQuit()
      redir => buffersoutput
      silent buffers
      redir END
    "                     1BufNo  2Mods.     3File           4LineNo
      let pattern = '^\s*\(\d\+\)\(.....\) "\(.*\)"\s\+line \(\d\+\)$'
      let windowfound = 0
    
      for bline in split(buffersoutput, "\n")
        let m = matchlist(bline, pattern)
    
        if (len(m) > 0)
          if (m[2] =~ '..a..')
            let windowfound = 1
          endif
        endif
      endfor
    
      if (!windowfound)
        quitall
      endif
    endfunction
    autocmd WinEnter * call NERDTreeQuit()