The cartels find it cheaper to hire the Mexican Army to escort drug smugglers north and, if intercepted, the Mexican Army puts up a blocking force to stop Border Patrol Agents from closing and apprehending. The drug smugglers go south and try again later. Before hiring the army, the drug smugglers would be intercepted and drop their load and run, costing the cartels a bunch, so, they hired the Mexican Army.
or they Shuck and Dive. They create a diversion in one location to draw the border agents in while the real shipment goes through another location