airline-tabline: the action of the first tab is different from others

I’m using the plugins Nerdtree and Airline in my vim. Here is my config:

autocmd VimEnter * NERDTree
autocmd BufWinEnter * NERDTreeMirror    " make Nerdtree available on each tab
let g:airline_theme='sol'
let g:airline#extensions#ycm#enabled = 1
let g:airline#extensions#tabline#enabled = 1
let g:airline#extensions#tabline#fnamemod = ':t'
let g:airline#extensions#tabline#buffer_idx_mode = 1

When I execute vim, I can navigate files on the original tab as expected. For example, I opened five files with the help of Nerdtree on the original tab and then I could see the tabline as below:

Then, I press t on some file in Nerdtree to open a new tab. On this new tab, there are two unexpected things:

1. I can see the name of Nerdtree is shown on the tabline, which isn’t shown on the first tab.

2. No matter how many different files I try to open, there is always only one filename is shown on the tabline, meaning that the new filename will replace the old one.

In a word, I want the new tab (created by pressing t in Nerdtree) to act just like the first one.

How to achieve this?