Was the site previously httP and has recently become httpS ? That could also be the cause (in combination with no HTML code change). Seems improbable as Google has been down-ranking non-httpS sites for some time, but maybe something has forced a switch to httpS for the main site
If I modify the Address from httpS to httP it immediately changes back to httpS ... of course I don't know if that is a new thing, or if it has been forced-httpS for some time. (Simple Machines version/copyright does not look like it was upgraded recently

, so not that. Maybe Hosting site has change to force httpS ...)
I can't find lots of people screaming on Google, so presumably local to this forum / or maybe Host