Same content, same competitors, have long been there in the index but never saw the light of day because our content was deemed better. Now, when ranked differently, they hit just the right spot.
New weighting of internal links, possible. But then everyone would be affected in the long tail field, thus canceling it out overall.
I don't think it necessarily cancels out. All sites are different, at the very least in the number of pages hence internal links, and so the effect may be quite different on different sites.
Could be Google is now crawling deeper so found many new links, that might be throwing everything out of whack.
I'm not sure if they crawl any different than they have been in the past but, with Caffeine infrastructure, they are supposedly able to take a better look at what's been there all along (and often is obsolete/erroneous by now). It may benefit older sites but it may not- if the old site had a lot of redirection errors/dropped URLs etc. - I have a couple examples like that.
Could be Site Speed was just introduced on more than a "test" basis.
In the aftermath of the announcement I've done some speed tests and looked at a variety of sites and came to a conclusion that this may be the last possible reason you need to be concerned with. Not saying it's not one of, just that you really need to exhaust all other possible problems (which is impossible in practice) until site speed comes into play. I'm talking about a site that does not take a whole minute to respond of course.
Could be lots of things. IMO best to wait for the dust to settle.
Don't lull yourself into passive waiting: just as success in Google breeds success via new links, viral promotion, bookmarks, feedback etc, the opposite is also true. Regardless of whether or not the site "deserved" the traffic drop in the first place, if you suffocate it without Google traffic long enough it will become irrelevant, obsolete and will further slip in ranks via "natural" ways - link attrition etc.