It has to do with breaking a Force Bond with a non Force User. When Anakin Skywalker changed into Darth Vader, his Force Powers he was using to manipulate Padme were ripped away from her through a Force Drain and that was the “death nail in the coffin”. She physically died by the Force Choke, but she could survive that being hooked up on machines. But once her heart was broken, her soul was ripped from her by the Dark Side of the Force for its uses and the coils of her life dissipated when the babies were born. She had no more will left to live.