The next time it happens, run a system diagnostic and call Sonos Support directly to discuss it. They should be able to determine where the command for raising the volume is coming from. Traditionally, this has been one of several things….a stuck button on a physical device, such as a phone, or something sitting on the speaker itself against the buttons there. Or even sometimes, if you’re a Spotify user, from a remote person using Spotify Connect to reach your system and turn things up by mistake (this “connection” can be cleared by playing anything that is not Spotify on your Sonos).
Let us know what you find out. Especially if it’s a software issue, but in the handful of posts I’ve seen about this kind of thing, there’s always been some other reason for it.