I think Arachnid (OP) may be onto something. Today I had a similar problem, and this is why I'm concerned.
I was not using Vivaldi. I'm in my writing app and just have Vivaldi up in the background with a few basic sites pulled up (google sheets, gmail, google calendars, zillow, and FB messenger). I am not actively using them. While writing in my separate app, I suddenly got a "download complete" pop-up notification from Vivaldi. It showed that I had successfully downloaded a .xlsx file about something to do with LOA and the source came from a .gov site. I cannot remember the last time i was on a .gov site. I panicked and deleted it from downloads and recycle bin immediately without recording the details, hoping it was a one-off situation.
Five minutes later, I'm again not touching Vivaldi. Haven't changed the sites on it or used them at all. I get ANOTHER "download complete" pop-up notification. This time, I had the presence of mind to take a screenshot.
78de9d77-54d6-419f-b1f7-4f4cfffe15ef-image.png
Now it's from a different .org site for Lassen County, CA (I assume, based on google's results for Lassen County, I did not open the website address). I live in the midwest and have not done any research on California. There's no reason for this document to be auto-downloading to my computer unprompted.
I don't know what's going on, but Vivaldi is DEFINITELY downloading documents unprompted. I'm less panicked since they are from .gov and .org sites so probably less likely to be malicious but at the same time, it's still very unnerving that this can happen.