Google is always experimenting. I think that Google's objective with all of its tests and changes is to improve user satisfaction with Google results, so that users will return.
Title rewrites, therefore, would be intended to increase clickthroughs on pages Google thinks are relevant for a query. Because they need to test variants to see which work the best, they've got to be wrong part of the time.
Thinking out loud... the new patterns you're seeing might depend on what it is they're rewriting. Yes, it used to be that if you had too many keywords in the title, they would drop the extraneous keywords, and the rewritten title displayed in the serps would include query-related phrases followed by the site name. My assumption is the same as yours... that they were including the search terms that would get highlighted before the site name, to attract the eye. This possibly might be something that Google is testing now. I don't know that for sure.
Long discussion where I make a bunch of observations about the old rewrite patterns is in this thread...
Google is now rewriting all my page titles Aug 2012 https://www.webmasterworld.com/google/4480232.htm
[webmasterworld.com]
This evening, I took a look at a site I'd once optimized (not the site discussed in the above thread, btw, but another one) where a new owner had either goofed up or thought they were improving things, and changed all the previously well-optimized titles to be the same as the home page title... so all the titles on the site were the same... a really stupid thing to do. For me, it's useful for studying Google title rewrites.
What I'm seeing now is that some of the old rankings, which were apparently driven by links and by onpage content, seem to have held. The onpage content was not keyword-stuffed, but was in fact really relevant content, so it's not a question of keyword density wins, eg.
The Google-rewritten title now displayed in the serps is perhaps an indication of why Google kept the ranking... the first few words of the serps title are anchor text of a global nav link for the page and confirmed but not duplicated by several external inbound links... and the remainder of the title is the title that matches the home page title, which has simply been shoved over to the right until space runs out. Does this resemble what you're seeing?
Something else?
I'm thinking that the pattern varies depending on circumstance.
The pattern is not always the
Summary Query Phrase - BrandA Widgets (which is perhaps the pattern you're expecting) that I mention in the eariler discussion... at least that's not the pattern in all situations. I don't know, though, whether what you're seeing, and what I just described, is a new pattern, or just different circumstances, or a Google experiment to add something additional.
