I'm pretty sure this trend comes from TV industry and heavily influenced by the internet.
Main reasons and reasoning, in my opinion:
1) Making trailers is an art. That epic narrator voice combined with sick editing skills produce, 90% of the time, trailers that are better than the movies themselves. In other words, you gotta make the trailer count as much as possible. Especially if your movie is not a blockbuster.
2) Primetime TV spots (or regular tv spots, when compared to online advertising) are very expensive. Either they show less of the movie or they show more of the movie and the movie name pops for a shorter time. It's a struggle.
3) You don't want to stretch or crop your image, so letterboxing on home TVs is inevitable. Thus, "empty/free space".
4) The target market is used to seeing the name of the movie/series right below/above the video, on youtube or other video sites.
5) Twitter and #hashtags. #nuffsaid
6) ...Which led the TV industry to start making overlays of hashtags for their TV Series. This DOES have an actual impact on viewing ratings, because the tag will be more memorable, will be used more, and then friends and followers will see that subject coming up more and will want to get into the conversation.
7) Movie industry saw in this a way to stick the name of their movie better in people's heads, as well as increasing volume of talk about the movie (especially on twitter). This is really important, by the way, when you want to release a new, original, non-billion-dollar-budget movie. It's hard for a newcomer to be remembered at the ticket booth when there's "Superman 8", "Xmen 5" and "Fast and Furious to the Moon" as your competition.
I have no idea about the official industry term, but I'd bet $10 that inside the industry it's something like "tag" (as in "I finished the trailer" "Did you remember to tag it up?") while to the target market it's something like "brand" (as in "a branded trailer has much higher retention and conversion rates, and lowers the overall cost of social media marketing by X%")
Forgot to explain my opinion on the conversion and trend:
As for the last two trailers (and all trailers that didn't use a generic white font over black background), unfortunately it seems that ONE trend is to call ever more attention to this (i.e. ever LESS attention to the trailer itself). This is far from the main trend, IMHO. But I think the reasoning went like this:
1) TV Stations started "tagging" their trailers for next episodes and such.
2) Movie industry started tagging their trailers for less popular movies. Possible reasons stated above.
3) Movie industry started putting movie title instead of a hashtags, because hashtags are hashtags. Kinda like how QRCodes are QRCodes. They kind of have their uses, but it should be way less than the current trend.
4) Movie industry saw possibility of branding the space, using fonts, colors and elements from the visual identity/motion design into the "header" of the trailer. Not just necessarily a good thing, of course.