I had an issue similar to this but it was more than just the volume popping up. I could actually watch my volume go to zero after it popped up. I would then use the buttons to turn the volume back up but it would repeat itself about every 10 seconds. And even worse, it would randomly call whatever the last number in my call log was. The random calls happened 3 times in a 10 minute span. Extremely frustrating when 911 was the last number I had called.
I started having this problem over the last 24 hours so I tried thinking back to anything I did in that time frame including Bluetooth devices and app installs. The only thing I could come up with was an expensive set of Bluetooth headphones I had paired at the office the day prior. Even though I was 50 miles away from the headphones I decided to go into the profile and delete (forget) it. Low and behold, my phone immediately stopped the volume pop-up and changes and no more random phone calls. I've had zero issues since.
FWIW, I have a Pro Max on 13.3.1. I would suggest removing any Bluetooth devices you have in your pairing list and see if that solves the problem. If so, slowly add them back until you have an issue but realizing you can't assume the one you're connected to is the one creating the problem.