Why does my iphone launch itunes when i lock it?

I have an updates iPhone 5, and I connect it to my car via Bluetooth to be able to make and receive calls through the car's sound system. However, even if I am listening to FM or other source different to the iPhone, whenever I lock the phone's screen, it automatically launches iTunes and the car connects to that audio source and my phone's music starts playing. It is very annoying... any help?