What triggers Diana's memory flashback in Act 1, Scene 4? What is the memory of?

After Gabe goes, Dan appears, dressed in clothes from years before that looks like the shirt Henry wears. The action flashes back to his marriage proposal, triggered by the news that Diana is pregnant. Stage directions suggest that Diana's memory has been triggered by watching Henry and Natalie, who realizes that Diana has been watching and hurries up to her bedroom.