CarPlay Conundrum

I'm hoping someone here smarter than me may provide insight into strange behavior between my car (Nissan Leaf), my iPhone, Spotify, and my Dutch and Dutch 8C (active) speakers. 

The 8Cs are connected via ethernet to my network and I can stream directly to them via Spotify. Love them!

When I'm in the house I can open up Spotify, select the 8Cs as the device or destination and start listening. 

And typically when I get in my car I expect CarPlay to take over (via USB or Bluetooth) and start playing music in my car. 

However, 50% of the time I get a few minutes from my house and I get a phone call from my wife letting me know the 8C's are playing. 

I have to manually go into Spotify and change the network or destination from the 8Cs to my car. 

I'm someone might be able to explain why this is happening and how to stop the 8Cs from playing when I get in my car. I would hate the leave the house for a few days only to find out from my neighbors that the 8Cs were blaring 24/7.