Starting on February 27, Apple will be requiring all developers to have two-factor authentication enabled for their Apple IDs, with two-factor necessary for signing into Developer accounts after that date.
Apple today told developers via email that the requirement is being implemented to help keep developer accounts more secure and to make sure that no third-parties can access a developer account.
Developers who do not have two-factor authentication enabled for their Apple IDs will need to turn it on by February 27.
Two-factor authentication can be enabled on an iPhone or Mac by following Apple's instructions. Once enabled, a verification code from a trusted device will be required when logging in to a developer account.