1) Twitter/Elon has made the conscious decision to censor all references to Mastodon in an attempt to retain market share.
2) Somebody shared something bad with a mastodon.social link, and Twitter's always-creaky moderation systems, now nerfed and defunded by Elon, completely failed to realize the implications and added mastodon.social to the same global domain ban list as viruses4u.ru and freeviagra.xxx etc.
My money is actually on #2, because the ban only appears to impact mastodon.social and not any other Mastodon instance, but I guess we'll find out once this hits mainstream news and Twitter has to make a public decision on this.