Zoe was in the UK when a couple stopped her to ask for help taking their picture. That picture turned into a marriage proposal, then it turned into a song, then everybody started singing to Zoe and she was very confused. And much like Bruce Willis at the end of "The Sixth Sense," there was a twist she didn't see coming.
Her boyfriend Sameh had actually coordinated all of this with the flash mob group Club Mob, as part of a marriage proposal to Zoe herself. She didn't even know he was in the UK, so she was very surprised and excited to say yes.