User accounts are locked if users exceed the maximum failed login attempts limit. Once locked, the account must be manually unlocked by a user granted the MANAGE ANY USER system permission.
However, since users with the MANAGE ANY USER system privilege are also subject to lockdown due to failed login attempts, you might have a scenario whereby all users with the necessary system privilege to unlock accounts are themselves locked out. The automatic unlocking of user accounts feature prevents this lock-down scenario from occurring.