I think it’s specifically from this post in the “Trump elected president” thread. From the looks of the code, it’s an embedded link from GIPHY that’s trying to serve a GIF that’s been wrapped in a Flash container. At a guess, the poster probably copied the wrong embed code (though why GIPHY has the option to share a GIF inside of a Flash container is well beyond my ken).
I’ve been getting download prompts for the same file in Vivaldi, which I use at work with Flash disabled. There’s probably some oddity to the way the Flash file is being served that’s conflicting with Apple’s newly-implemented Flash restrictions to cause Safari to download the file instead of blocking it from displaying.
