Trap
She knew she'd trapped him into the engagement, and she wasn't proud of it
Even after all their years of dating, they'd failed to discuss marriage. Glinda had dropped multiple heavy hints, but he always dodged the discussion.
Finally she decided she would have to be the initiator. And as manipulative as she knew it was, it had to be in public, so there was no chance he'd turn her down.
All she wanted was to be certain he wouldn't leave her, couldn't leave her. She already lost Elphaba, she didn't know if she could stand losing Fiyero as well.