Credential attacks against administrator accounts
Attackers try stolen and guessed credentials against login pages continuously, because it is cheap and it works. Reused passwords from unrelated breaches are the most common way a small-business website is taken over.
The countermeasures are unglamorous and effective: unique passwords held in a password manager, multi-factor authentication on every administrative account, prompt removal of accounts that are no longer needed, and rate limiting or lockout on login endpoints.
