This has happened because people who's first language is Spanish, have moved into the U.S. and since at first not many of them new English over a period of time some of the Spanish words became a part of English, and some of the English became a part of the Spanish.
The mixture of the languages is common when 2 or more are mixed in one area, so everyone tries to understand each other by putting another language within their own when speaking.
I hope this helps :)