It's so obscure now, I actually had to go back and reread what was going on there at the time. On May 29, the mainstream consensus was how Italian populists were screwing up Europe. Global bond markets were supposed to be selling off, massacred as global recovery took hold. Instead, worldwide the most liquid, safe instruments had been hugely bid for several weeks culminating with heavy buying on that particular day.
The new Italian government had tried to appoint euroskeptic Paola Savona as Finance Minister, a nomination which immediately triggered an EU backlash from Brussels.