Usually when you have an ad in Bing Ads, you get to choose your ad copy description that appears in SERPs. In some cases, however, it looks like Bing is taking descriptions from landing page text, meta tags, and elsewhere. Furthermore, these descriptions are "truncated by Bing Ads, and truncated in multiple spots depending on the advertiser and where Bing is pulling the description from."
It's worth emphasizing that, according to this story, it appears to just be a "test" affecting some ads and not all. Furthermore, it's only been noticed on Firefox and desktop - Chrome, Safari, or mobile browsers aren't seeing it. So don't go crazy and panic yet! It could be something worth watching though.