no, nobody "explained" it. Todd Howard in an interview said that "everything that happened in previous games, including New Vegas, happened [in the show]", and that the bomb falls [in Shady Sands] just after the events of New Vegas", and when asked if "the Fall of Shady Sands doesn't necessarily mean the nuke", he said "right". so, none of that explains what the "Fall of Shady Sands" is supposed to refer to if it doesn't refer to the very obvious possibility, and it doesn't explain how Shady Sands could have "fell" about 4 years prior to the events of New Vegas and absolutely nobody in the Mojave Wasteland had heard about it.