I think this most often happens when the submitter for some reason didn't get the login screen. The NYT seems to be actively varying when it prompts people, in order to keep links to their stuff included in "the online conversation". Some cases where there's no prompt: 1) I think anyone from an .edu IP range; 2) if you follow a link from a big link-sharing site like Reddit (or Google News); 3) if it's your first nytimes click of the day.

HN seems to not be big enough to get the #2 treatment, so people who find a story on Reddit or Google News and then submit to HN might not notice that it went from no-prompt to prompt-required. People who hadn't read any previous nytimes stories that day might also not notice. More info: http://www.businessinsider.com/janet-robinson-online-paywall...