User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130710153323/@comment-3146930-20130711194815

Thanatos was captured some time before Jason Grace was taken by Hera/Juno, about a month before. This caused chaos in the Underworld as Thanatos was no longer protecting the borders of life and death, leading to a large amount of escaping souls. During this time, Gaea also took control of the Doors of Death, causing monsters to start to reform instantly. Nico used all this confusion to slip Hazel out of the Underworld, but he didn't use the Doors of Death as there are other ways out of the Underworld.