When you open a file node in NERDTree and if that file is already open in a buffer in another tab, NERDTree switches to that tab to activate the file. Since I find this tab-switching behavior rather jarring, I use this plugin to alter NERDTree's file open behavior so that it will open the file in the current tab instead of switching tabs.

Use the following commands:

cd ~/.vim/bundle
git clone https://github.com/mortonfox/nerdtree-reuse-currenttab.git

Add the following to your vimrc:

Plugin 'mortonfox/nerdtree-reuse-currenttab'

Install with :PluginInstall.

Copy the reuse_currenttab.vim file to ~/.vim/nerdtree_plugin/ (*nix) or ~/vimfiles/nerdtree_plugin (Windows).