I believe MS declares Q1 earnings today, and there had been some rumblings that they were risking accounting / reporting liability by failing to characterize their material OpenAI stake.

What probably happened:

   1. MS's accountants raised a warning
   2. Existing agreement prohibited disclosure of terms
   3. MS told OpenAI that wasn't acceptable and MS needed to publicly report details today
   4. OpenAI coordinated release of this, to spin the narrative