Essentially, Trump's promise to build a wall and end illegal immigration
has caused a rush for the border that is crippling Mexico's resources.
Mexico can either let undocumented migrants enter their country willy
nilly en route to the US, or they can take strong action to stem the
tide of immigration for their own benefit. Which would be best
accomplished by building a wall, either literally, or through a strict
enforcement policy.
All of a sudden, Trump's absurd claim that Mexico would pay for the wall doesn't sound so absurd.
http://americanactionnews.com/articles/how-donald-trump-may-already-be-forcing-mexico-to-pay-for-the-wall