autocmd VimEnter * NERDTree
autocmd BufEnter * NERDTreeMirror
edit: The above command seems to open the new tab in NERDTree's buffer. Instead use this as mentioned by wejrowski in the comment below :
autocmd BufWinEnter * NERDTreeMirror
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…