The reason? Trump is declaring an "emergency" under the "National Emergencies Act," which, he says, authorizes him to spend U.S. taxpayer money on the wall without congressional authorization. He's going to have the U.S. military, which will dutifully follow his orders, construct his Berlin Wall.
Trump's action is the very essence of dictatorship. Check out other dictators around the world — Maduro in Venezuela, Ortega in Nicaragua, Diaz-Canel in Cuba, Kim Jong-Un in North Korea, el-Sisi in Egypt, and Zi in China. They don't have to jack around with congresses. They have the authority to just act or order. That's what makes them dictators.