A system or account is infiltrated. This is done using passwords that were lost during a previous data leak.
The internet is full of lists with login or password combinations that have been leaked from somewhere. In most cases, they are the result of data leaks from online services. The data may also stem from phishing attacks. Attackers collect these lists, reconfigure them and use the passwords to launch automated attacks on other services in the hope that someone has used the same password for several services. For example, if the attackers are in possession of the access credentials for your email service, they will use the same login data to launch automated attacks on webshops, online banking services or even corporate accounts.