Essentially they didn't believe him because his English is poor

but he was born in the United States. He just spent a lot of his childhood in Mexico. But considering he had several forms of legitimate identification, it's a shame that they pressured and deported him.

Wrongfully deported American home after 3 month fight: U.S. birth certificate wasn't enough to persuade border authorities

A Texas-born U.S. citizen who was detained, questioned and deported to Matamoros, Mexico, in the middle of the night has been allowed to re-enter the United States, ending a nearly three-month ordeal.

Luis Alberto Delgado, 19, was carrying his American birth certificate, Social Security card and Texas ID when he was pulled over in a routine traffic stop on June 17, according to Houston immigration lawyer Isaias Torres, who represented him in his legal battle for repatriation.

A South Texas sheriff’s deputy who apparently believed the documents were not authentic handed Delgado over to U.S. border agents. After eight hours of questioning, Torres said, Delgado felt pressured to sign a document agreeing to voluntary removal from the country and waiving his right to a lawyer. The Border Patrol then drove Delgado to Matamoros and left him, he said.

Granted, he did sign a document allowing them to deport him, but I'm not sure how much he understood about what was happening. At nowhere in the article did it mention whether he was given an interpreter. There are plenty of citizens out there who have broken English at best (and not just Spanish-speakers), and although that's a debate in and of itself, it's no reason to throw someone out of a country where he was born and had a completely legal right to be working and living.