iPhone defaulting to Music instead of Spotify

[straes]

This is perhaps a strange question, and I apologize if it has already been addressed.  I'm  not exactly sure what terminology I should use to explain this question.

Basically, I use my Spotify account all day on my iPhone 4S (iOS 7).  My wife typically only uses it on her phone a few times a week.  I can get out of my truck, not use my phone for 30 minutes, and when I connect it back in my truck, it starts to play the default iPhone Music app (which has no songs).  My wife can not use Spotify for days, and it still connects and remembers that she was using Spotify and not the iOS Music app.

Am I missing something?  Doing something accidentally that I'm not recognizing?

This is just annoying and I am trying to figure out why it is happening.  I understand that there isn't any way to specify that Spotify be the default app, but the timeout/default thing is bugging me.

Thanks.

[Peter__]

Is your wife still on iOS 6?

Spotify are investigating issues with iOS 7 and in-car systems on this bugs topic. 

Peter

[straes]

No, she is on iOS 7 as well. Her phone is a 5 though.

Also, just thinking about it now based on your comment, the vehicle has nothing to do with it when you boil it down. I use Bluetooth but the issue also happens if I'm just playing through the phone's speaker or through ear phones. The vehicle is just where I notice it the most because I use it the most.