It's most likely just part of the due-diligence required for the merger. Yahoo brass probably decided that publicizing this information wasn't worth the PR hit

It's most likely just part of the due-diligence required for the merger. Yahoo brass probably decided that publicizing this information wasn't worth the PR hit, and so they buried it, but Verizon doesn't want to take on the risk of a potential lawsuit, so their lawyers required Yahoo to disclose it if they want the deal to go through.