Trending design ≠ efficient design. Lot of me-too designers try to copy what's on popular websites even though it's clearly a wrong approach and that's how bad designs become viral (Jumbotrones is another example which has become viral but has measurably lower engagement metrics). Also many designer falls in to side-by-side measurement trap while what they should be measuring is users engagement metrics. Card layout looks pretty in PowerPoints and SBS numbers would generally shine but any designer who have actually measured actual engagement metrics with card layout would agree that it's horrible idea. BTW, I'm taking about card design where there are more than one horizontally laid-out cards.
Didn't say it was the right thing to do but that is surely up for debate though it is definitely a current design yes-yes. Personally, I like the card trend. It makes designing for responsive a bit easier and I enjoy the material looks which plays well with "cards'.
Pinterest is mainly focused on pictures and its users like to scroll at leisure through long lists, collecting items that catch their eye. On the other hand, G+ is more focused on reading single articles: notice an interesting headline, read the first few lines, click to read more.
This distinction between grazing for images and hunting for text articles is why lack of horizontal alignment makes Pinterest more enjoyable and G+ harder to use.
