What happened to Siteground is that its DNS for servers that run on AWS were affected which made it difficult for Google to crawl sites hosted on SiteGround. These led to thousands of sites hosted on SiteGround experiencing loss of traffic.
The most affected were eCommerce sites since products on Google shopping were removed and Google Ads campaigns stopped because Googlebot could not crawl them.
The issue has since been resolved.
You can follow this Twitter thread to understand more about what went wrong.