In 13th century Vietnam someone overthrew the Ly dynasty and forced all the members of that family and its nobles to change their name to Nguyen.
When that dynasty was overthrown, its supporters changed their name to hide from the next dynasty.
This happened several more times and as a result, nearly 40% of Vietnamese have the name Nguyen.
Internet says the name derives from a Chinese name "Ruan." Ruan was a place fairly north in China but its people were displaced and moved south. There is an instrument with the same name.
Why was Nguyen chosen to be the loss penalty for the defeated dynasty? If I go by the etymology the internet claims, perhaps the new king was trying to drive home the point that his quarry were really foreigners who drifted in. At one point, the founder of the Ly Dynasty's founder was from what is now Fujian, China, but not Ruan. It would seem that if he wanted to invoke the foreignness of the Ly, he should made the people change their name to Fujian.
Does anyone have a good answer to what the name means and why the King chose it?