"Mexico is considering a retaliatory tariff against the United States over delays at the border that it is blaming on the reassignment of agents from customs inspections to dealing with the dramatic rise in migrants seeking political asylum.
The Mexican government says that U.S. efforts to step up the number of agents focused on migrants showing up at the border has left customs checkpoints understaffed and drastically slowed trade.

“We cannot allow this kind of unilateral measure by the United States to affect us,” Mexico Foreign Trade Undersecretary Luz Maria de la Mora said, according to the Mexico News Daily."

The second paragraph says it all. We are using agents to stop illegal immigration which Mexico encourages. When we slow it down they stay in Mexico and cause problems for the Government there. they (Mexico) wants the trouble makes to move on into the US.
Why don't they stop the illegals on their southern border rather than letting them swarm on through to the U.S, border.
I have a feeling that if they put on a tariff it will not go well for Mexico. Just a hunch.