Why is it, "Je m'attendais à ce que tu me demandes en mariage"

Bonjour Diane, Haha, I agree that it's probably not what you expect to hear ! In this context, rather than dread, this sentence could also reflect disappointment, like "I expected it, but you didn't.". Though to be honest, the main impression is definitely something like "Pffew, I was scared, I thought you were going to propose."!

If you said "J'attendais que tu me demandes en mariage." would be a more literal "I was waiting for you to propose.". To express the hope of a proposal, you would use another verb altogether, such as "J'espérais que tu me demanderais en mariage." (I hoped you'd propose.)