I have a bluetooth headset that I use for an Android phone and a MacBook. It has the feature of being able to connect to both at the same time, and switch to whichever one is actively playing. There is no mixing, the audio is just "stolen".
However, when some audio applications idle, they will "steal" the audio. So, if I'm using my phone, and the MacBook will come into range while Spotify is open (but not playing) it will pause playback on the phone and switch to the MacBook. This is because once the headset is in range, macOS automatically connects to it and then switches the default audio device to it.
So far in Big Sur I've not found an option to disable it. This question has a solution that works for previous versions but not Big Sur.
It will even switch audio devices if I have wired headphones connected!
Is there some workaround I can use for Big Sur?