From October 2013 to September 2014, for the first time on record, fewer than half of the immigrants apprehended at the US/Mexico border were actually from Mexico. The majority of them came from somewhere else.
The proximate cause for this is the Central American child and family migrant crisis of spring and summer 2014. Tens of thousands of unaccompanied Central American children, and parents with children, arrived at the US/Mexico border and presented themselves to Border Patrol agents — overwhelming the federal government's resources and sparking a panic about border insecurity.
Border Patrol statistics indicate that about 20 percent of the non-Mexican immigrants apprehended in 2014 were unaccompanied children from Guatemala, El Salvador, and Honduras. Additionally, Border Patrol apprehended 68,445 "family units" — that adds up to at least 137,000 more immigrants (and probably more, since many families may have more than 2 members), though it's not clear how many of those were from Mexico. In all, it's reasonable to assume that a majority of all non-Mexican immigrants were Central American children and families.
But while the child and family migrant crisis is undoubtedly the reason for the turning point in apprehensions, it itself is a culmination of a couple of broader trends. First of all, immigration from Mexico has been extremely low since the recession began in the late 2000s. In 2011, net migration from Mexico fell to zero. Unauthorized migration fell even further: the Pew Research Center estimated that 900,000 fewer unauthorized Mexican immigrants lived in the US in 2011 than in 2007.
The drop was undoubtedly driven by the recession, but it's possible that immigration enforcement has also played a role — especially because, under the later years of the Bush administration and throughout the Obama administration, the federal government has ratcheted up penalties for Mexican immigrants caught at the border.
The two trends are linked. There's evidence to suggest that when human smugglers and traffickers found that Mexico wasn't a profitable market for them anymore, they turned their efforts to Central America. And since Central America has some of the most violent places on earth, they found a ready market of people who were desperate to escape — and who, in many cases, would qualify to stay in the US legally after arriving here and applying for asylum. In particular, it's likely that smuggling and trafficking networks increased their capacity from 2013 to 2014, and that this was a major cause of the spike in Central American migration from last year to this spring and summer.
The US government has aggressively responded to the migrant crisis, and it claims to be working with Mexican and Central American governments to target smugglers (though government officials are warning that migration from Central America might surge again this spring). If they succeed, Central America might become as unappealing a market to smugglers and traffickers as Mexico has been — which raises the question of where they'll look next.