Sites are delisted from Bing & DDG as a result of negative SEO attacks. I've experienced this multiple times and responded to a couple of threads on HN about it.

The attacker doesn't care about your Bing listing, it's an unintended coincidence of an attempt to get your site deindexed from Google search that doesn't work.

Google ignore bad links to your site, Bing don't.

After two years I can confidently conclude that MS have done a great job of hiding this 'defect' and virtually nothing regarding the root cause.

In my experience, the sites come back on their own eventually but with a lot of really weird URLs being indexed first (a sure sign of the original attack).