The biggest single factor, in my opinion, is economics. Just as the strength of the American economy and the corresponding weakness of Mexico's economy drove hundreds of thousands of Mexican nationals across the border to the United States, the recession in the US and the corresponding growth in the Mexican economy (its GDP growth is expected to outpace Brazil's this year) has greatly eased the pressure at the border. When individual states like Arizona and Alabama pass strict enforcement measures as well, it adds to the reverse flow of immigrants back to their countries of origin.
So we're mainly talking about economic incentives and political and social disincentives. Many citizens of Mexico (as well as other countries in Latin America) find it economically beneficial to return home at the same time as they find tougher legal and social pressure in the US for them to leave. The result is a net immigration loss for the US, meaning more Mexican nationals have left the country in 2011 than came into the country. Take those statistics with a grain of salt, however, as undocumented populations are notoriously hard to quantify.
It is also important to note that the Obama Administration has much more aggressively deported undocumented immigrants compared to previous administrations.
I believe the answer lies in three major factors, ranked in order of importance.
1. Economic slowdown in the United States. The primary motivation for migration to the United States from Latin America is driven by the availability of jobs, and in a recessionary economy less of those jobs exist.
2. Lower birthrates in Mexico and other Latin American countries. The lower birthrate means there is less pressure on poor families to send young workers to other countries where they can find work and send money home to support the family.
3. Increased deportations and border security by the Obama administration. Deportations are up and while taken by themselves they are not a significant factor in decreased migration, as a whole the firmer pressure by the United States leads many experts to conclude that migrants are staying away in unknown numbers.
The linked PDF below contains the actual study which recently and thoroughly documented this trend.