BufLeaveIt starts when changing to another buffer. Neither <C-z>switching to another Terminal.app tab will cause this, because you are using the CLI Vim, which does not care about the GUI of the terminal emulator or the environment and ... you do not leave your buffer.
FocusLost ( , , GUI, , , ).
, , , MacVim, CLI Vim.
, Vim , , , CLI Vim.
~/.vimrc autocmd FocusLost * :wa, if has("gui_running") , inoremap <Esc> <Esc>:w<CR>, ESC. , .