I'm pretty sure there are a few concepts at play here based on my experience on the web since '98 to present (2013).
Back-in-the-day, and still a little bit now, people were really concerned about content appearing within the viewport...all of the content. "Users won't know how to (or that they have to) scroll!" They would scream. Or, it was just annoying back then because scroll wheels weren't as prevalent - "You mean I need to use arrow keys, or drag the scroll bar to see all the content - forget that, make separate pages, add some buttons." And so on.
With sites like Facebook (infinite scrolling), the serious prevalence in the marketplace for scroll wheels, and Apple recently ditching constant scrollbars entirely...people get it - for the most part - even if there isn't some indicator to tell them to scroll they will still try, just to make sure (this is why Apple scroll views bounce).
HTTP Requests and Mobile
With so many people hopping onto your site with a mobile device - it's just friendlier to do a page in a single view - in a lot of cases to cut down on the number of HTTP requests to view all the content you're interested in, and not have to wait for the page to load every time you hit a link.
Also, with AJAX you can load smaller pieces of content on-demand (think Wikipedia for mobile). While this does not cut down on the number of HTTP requests (probably increases them in fact) - a user doesn't have to use their bandwidth downloading pieces of the content (s)he really doesn't care about.
Number of Pages Does Not Equal a lot of Content
For the first few years I was talking web design to folks I would do content audits: How much is really there? Then ask the question which this raises. Why do you have so many pages?
One particular site I worked on had a link to Emergency Numbers. This was a laboratory environment; so, the types of emergencies ranged from chemical spills, to fires, to radiation leaks. Each type of emergency had its own page...with 2 phone numbers each. So, picture this, you're on fire - your lab-mates run over to the computer (because this is likely to happen) - you get to the main page of the site, click Emergency Contact - then you have to read what type of an emergency you are actually having - then click the link - then get to the phone number.
When I asked why these weren't on one page, the response was, because users will just pick the first number they see. When I asked why there wasn't just one number to take all the calls and then route them to the proper specialists (a la 911), there were crickets chirping.
The links you provided as example, are good examples of this concept. Grab all the text on the page (actual content). And check the word count.
It's typically easier, to quickly edit a single HTML page - or database entry - than 5 related entries. And remembering they are related. Etc. Etc. Etc.
So, while I think it is kind of chicken and egg whether the upward trend for these types of sites is based on the emersion of the "960 grid" and other tech. or if the tech. became popular (were created to accommodate) these other factors - I do find it pretty interesting as well.