W1zardK1ng
First of all I want to just say that I 100% don't know, I'm just guessing.
But, I would imagine it has to do with the fact that the phone app had moved to the background. Maybe at that point the app could no longer do something, like access the microphone. It might not even be the phone "app" that's using the microphone, it might be the Telephony service (or something).
I personally don't talk on the phone, so I can't say whether the same happens on my phone or not.