I believe this is a response to a transformation that is going on now. Cities are becoming more attractive. Growing wealth leads to gentrification which is turning ugly neighborhoods in classy areas. In New York City - where I live - you can see that in Brooklyn, Queens and other parts of the metropolis. New York City and other U.S. metropolises enjoy a sinking criminality and the environment is getting better thanks to stricter environment laws and better technologies (less car exhausts, less industry, more service providers).
The main reason that YM migrate to the cities their parents have left: Jobs. Big cities have plenty of companies and public employers. Thus it is much easier there to find a job, also to switch to a new employer.
YM also may want - and have to - achieve more education. The global economy is changing faster and faster demanding more knowledge and skills. Metropolises like New York City are home to a lot of Universities and colleges.
It also is apparent that YM want to have a lot of fun. In big cities you can see crowds of YM around the plenty of nightclubs, cafes, bars, cinemas and other leisure places. The high density also makes it easier for them to host and to attend parties.
Furthermore I have the impression that YM are hungry for culture. Cities like New York have a huge advantage. I spot a lot of young people in places like MoMa, Guggenheim, Metropolitan Museum of Art and other collections. Young people are also flocking to theaters and opera houses.
Big Cities have still disadvantages, of course. The main drawback are high estate prices and rents. On the other hand their citizens can save a lot of money because they don´t need to commute long distances and subways and buses are cheap alternatives to own a car. Filling the gas tank costs now 4-times you had to pay in the 1990s. Thus high gas prices are another reason to migrate to cities.
There also may be a snow ball - or networking - effect. As long as YM move to the cities they will find more of their kind there, good for friendships, dating, marriages. Thus I believe the trend will continue and more YM will migrate to the city in the coming years. Welcome.