It almost makes me wonder if they're doing this to promote CarPlay.
But then I'm reminded it may be related to changes that occured along with the new Music app, which I dislike.
It's got to be the changes related to Apple Music. Even when I'm connected via USB I'm having problems.
For example, I have a Playlist which is "Purchased". However, if I select any song from that, only one song ever plays. Why? Because the only song on my iPhone 5s that is in the Playlist is that song. All the other songs are in the cloud, and therefore can't be played.
I got around this by signing out of the cloud. But even then, things are just generally more glitchy over USB than they used to be.
Oh and I generally just don't like the new interface of Apple Music. Apple has really screwed up this release IMO, and I'm not even talking about all the severe problems people have been having with Apple Music subscriptions. I don't have an Apple Music streaming subscription, and after reading about all the problems, I definitely won't be partaking, even for 3 months free.