I can't remember where but I read clues about NetworkManager could be involved:
Sometimes NM starts fast enough to initialize the "lo" net interface but this does not create an event in upstart. Sometimes the script that usually inits "lo" runs first and generates the event.
Something else I see is different scripts just don't start because they are complaining about an IO error accessing the console. Another clue which could also explain why the loopback may not start? At least it relates to why Xorg does not start for me sometimes with an IO error accessing the console in Xorg log.