When your mom/spouse/friend leaves you alone in the room with a person you just met and it's getting way too awkward. Default friends are usually started if you know the person who left you is going to be gone for a while.
You start to talk; and gee, what do you know- a Default Friend has been made!
I left Barbara in the kitchen with my new friend Bobert while I went out to mow the lawn. By the time I was back, they were already exchanging numbers! What Default Friends.