This is currently happening for some collections e.g.
https://www.nexusmods.com/games/newvegas/collections/hss8nv
https://www.nexusmods.com/games/newvegas/collections/jscbqj

To reproduce:

• Install one of those collections fully (should work just fine on the first run)
• Delete the mods, but keep the archives
• Resume the collection installation - the collection installation will eventually hang on "waiting to install" with no active/pending installations queued