I had a similar experience yesterday. I’d stuck with Ventura because of worry that some of my apps wouldn’t run under Sonoma. I’ve been having some nagging little problems with my 2019 iMac, so I restarted it yesterday. I figured it would take awhile, so left it to do its thing. I checked on it a couple of times, thinking it was taking longer than usual. When it had finally finished, my OS had been updated to Sonoma. I’ve been a Mac user almost since day one, and this is the first time Apple has done something like this without my approval.