Kneeling to propose
No one knows exactly where this tradition is coming from. Some might say that it all dates back to medieval times when knights knelt before ladies. Others might suggest that it was a sign of surrender during feudal wars, where men did it as a symbol of surrendering their will and fortune to their beloved.
It might also stem from the Person tradition, where someone would prostate on the ground in order to show respect. Even if the roots of this tradition go back for many centuries, kneeling to propose is, even more, a pop-culture thing, as it became a standard for Westerners initiated in the 60s.