Pop the question
Definition 1 of 1
Pro Tip 1/3Romantic Setting
The phrase often implies a romantic and special occasion when someone proposes marriage. 
On their romantic getaway, John finally popped the question.
Pro Tip 2/3Surprise Element
It suggests a surprise or unexpected moment, though it might also be anticipated by both. 
She had no idea he was going to pop the question right then.
Pro Tip 3/3Casual Idiom
The phrase is casual and idiomatic, more often used in spoken English than in formal writing. 
At dinner, he casually popped the question and she said yes!