It appears the truck was sold to someone in Mexico and the driver was hired to deliver it. He was probably paid to bring them back across the border and deliver to the WalMart parking lot and then on to Houston
They do use trucks that is for sure. Here is an example
from back a few years and they paid $7,000 each to ride.
Mexico City (CNN) — Mexican authorities said they found and detained more than
500 illegal immigrants from Central America and Asia who were crammed inside
two tractor-trailers heading toward the United States on Tuesday.
X-ray equipment detected the immigrants at checkpoints in the southern state
of Chiapas, the state attorney general’s office said. The drivers of both
trucks sped away after authorities ordered them to stop, but police apprehended
them nearby.
Inside the tractor-trailers, they found 513 people from El Salvador, Ecuador,
China, Japan, Guatemala, India, Nepal, Honduras and the Dominican Republic.
http://www.cnn.com/2011/WORLD/americas/05/17/mexico.migrants/index.html