Since 2010, the iTunes Store has offered 90-second previews of music for tracks that are longer than 2:30. This was done because 30 seconds simply isn’t enough to sample music. But suddenly, many tracks on the iTunes Store are only offering 30-second previews. I checked a number of albums, and found that many have 30-second preview, though there are also albums where this is a mix of 30- and 90-second previews, and some with only 90-second previews.
For pop music, there is a mixture. On Pharell Williams’ Girl, seven tracks have 30-second previews, and three have 90-second previews. Ed Sheeran’s X, however, has all 90-second previews.
On the other hand, all 40 tracks on the new CSNY 1974 album have 30-second previews.
In the classical section, I checked a handful of albums, and they all had 90-second previews, except for tracks shorter than 2:30.
My money is on a server glitch on the iTunes Store. If all tracks were changed to 30-second previews, then I’d wonder why Apple was making this change. But since that’s clearly not the case, I think it’s best to wait rather than assume that previews have been changed across the board.