Could it be that this is a sort of branding? Or test of branding?
I'm seeing results that partially fit the branding model, but only if the title is too long.
I happen to have an interesting but limited test bed... let me describe. A while back, a longtime client and a well-known niche brand (let's call them BrandA) was acquired by a 900-lb gorilla (let's call them GorillaGuys). I've had more or less no input into subsequent SEO, but I've continued to watch the site over various Google changes. BrandA's site has been left operating, essentially unchanged except for news about the acquisition, changes in contact info, etc. Until recently, there's been no visible SEO presence on GorillaGuys.tld for BrandA terms.
Some months back, in the /Products/BrandA/ directory, GorillaGuys essentially cloned the BrandA optimized pages. They added GorillaGuys identification via a few extra headings up at the top of pages, but kept page titles unchanged.
Because BrandA's name was short, I'd always included it as the first word in page titles, so BrandA is being promoted as a branded product line of the larger company. Gradually, I've noticed the clones of the BrandA pages on the GorillaGuys site moving up to page two and the bottom of page one. BrandA pages continue also to rank, usually in the top two or three. Queries are brand name independent.
What I'm seeing in the serps is that on pages where my original titles were less than 64 characters, Google is displaying page titles on the GorillaGuys.tld site with no change.
On the few pages where I'd let the BrandA titles go long, though, say 80 characters (keeping the essential information in the first 64 characters but otherwise allowing extra descriptive phrases), on the GorillaGuys pages, Google has truncated the title endings and
added "GorillaGuys" to the ends of the displayed titles. It has not added GorillaGuys to the shorter titles.
This raises the question of whether the rewritten titles others are seeing are longer than 62-64 characters.
MUCH worse on Google UK Mobile search than regular Google UK search results. ie. some of the page titles aren't rewritten on normal search.
This too suggests that length may be a factor in what's rewritten... at least in this round of results... and that Google may be testing various default lengths for mobile display. The examples reported also suggest that Google is considering brand name an important term to be included... but perhaps Google considers this only when it's making a change.
It would be interesting, as some mobile devices display longer titles, whether Google may come up with some sort of adaptive title-length rewrite algo. ;)
I'm assuming that this, as with everything else Google, is an ongoing test in progress.