Car Play and Data Usage

On a recent 7 hour trip I used CAR PLAY simply to listen to Podcasts and music ON MY iPHONE. I did not open the MAPS app but I think it may be using data in the background if at some point I wanted to use the app for navigation. But for the 7 hours my Verizon cellular data use was over 900MB. I have no idea what caused this. It exhausted my Verizon Data limit of 2GB. The prior day of the trip I DID use data because I was running the SiriusXM Internet Radio app to stream music. That day I used over 800MB and that is why I only ran apps on my phone the second day.