Allowing Siri access from the lock screen is a potential security threat that has, in the past, allowed someone to access personal data like photos. Additionally, Siri will respond to any command given by anyone when your device is unlocked.

Apple thinks it has a solution, using a combination of a special phrase and authenticating your unique voice to ensure that Siri responds to you and only to you …

Patently Apple spotted that Apple has applied for a patent for this solution.

Up until now when a digital assistant has been invoked with a voice command, the digital assistant is responsive to the speech itself, not to the speaker. Consequently, a user other than the owner of the electronic device is able to utilize the digital assistant, which may not be desirable in all circumstances 

Apple's invention provides a method to train Siri to know the voice of the owner of the iDevice before responding to a command.

Source: 9to5mac