If you store WordPress code on Github inside a public repository all your code is public. Most people know and accept this, but people either don’t realise that their database credentials get stored inside the php file wp-config.php or they do and forget to exclude it from their repository.
This means that if you do a simple Google search for site:github.com master/wp_config.php password blob DB_PASSWORD you will find a whole rake of folk with their passwords on display.
It is impossible to tell if they are just sample details or real details, but it is still crumby practice and if you do it you really need to stop now.
I will be contacting people I find to let them know, but please pass this message on to any WordPress/GitHub users you know.