You may try Windowed-Borderless mode. This is visually the same as Fullscreen and it's what I typically run games in (there are situations where this can be undesirable but there is a very high chance you won't notice any difference).
As for why it's doing it... perhaps a keybinding on the controller or more likely background software of some sort. Perhaps even something like Win10's notifications?
Is the sound cutting related to when it switches to windowed mode or is that an unrelated issue?
The sound cutting happens when the screen windowing happens
borderless does remove the screen going small but not the sound cutting out.
Are your sound settings in the game set to the right option? I mean like Surround vs Stereo? I have 4 speakers but by default my computer enables 5.1 but I have no center speaker so I miss sounds, I have to manually adjust it in the SoundBlaster software to disable the center speaker then everything works well. Perhaps something like that?
You may try updating your sound drivers. If you are windows 10, windows update probably already does a decent job at updating them though. The drivers would likely be found at either your motherboard manufacturer website or computer manufacturer website (i..e. Dell) if it's a factory machine.
You may try Right Clicking on the speaker icon at the bottom right, Playback Devices, Properties on Speakers, Advanced. Change default format to CD 16bit 44100hz if not already set.
If you have audio software running (likely bottom right something like DTS, Doby, Soundblaster etc). It may have a certain mode enabled by default that isn't playing nice. You may try defaulting it or disabling any modes enabled or straight up ending the programs task and then test.
If you get your audio from your HDMI/Display Port the audio drivers are tied to your GPU drivers.
If you have a USB headset around to test with, you may try that to see if it also causes the issue. A USB headset won't use the computer's sound card (the 3.5mm jack kind will).
Good luck
Changing the settings seems to have fixed things. Although it has been sporadic in the past. I guess only time will tell. Thanks a bundle XD
