Vim itself and many of the plugins you use will configure autocmd triggers.
This are commands that get executed when a certain editor event takes place in
certain types of files.
A common example is having a tool like prettier configured to format the
contents of JavaScript files when the BufWritePost event happens, that is,
right after the buffer gets written.
You can list each autocmd that is configured for your current buffer using
:au and a couple arguments.
:au * <buffer>
The * says match against all events. The <buffer> filters them down to just
what is configured for the current buffer.
If you just wanted to know about BufWritePost events for the current buffer,
try this.
:au BufWritePost <buffer>
Or if you want to know about buffer number 4:
:au BufWritePost <buffer=4>
See :h :au for more details.