User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5608262-20121027154529/@comment-4876250-20121028125412

It might be Frank becase Mars told him that there will be a choice that Percy will have to make that he won't be able to, so Frank will have to do something. So maybe Nico needs to close TDoD, but Percy won't let him, so Frank hurries in and closes them. I'm only accepting of this theory if Leo gets Hazel. Otherwise, Frazel all the way and Nico can close the doors.