"Safari" would like to control this computer using accessibility features
Based on the way accessibility features work in macOS, it is reasonable to assume that when saving or entering usernames and password using Apple's Keychain, or when using AutoFill to enter data into a page, that it uses some type of virtual keyboard that allows the system to enter text in user land. This would be very similar to how assistive devices function when using an Automator extension such as the "Watch Me Do" function. The reason why this comes up is because it is attempting to either read or write to the screen on behalf of the user. 

This is an assumption, but it is based on working technical experience with the underlying software sets involved.