Very simple actually. Houston, like every major urban area in America, has an abundance of a certain ethnic group which votes demoncrap in excess of %100
Racial composition: 2010 Hispanic or Latino 43.8% White (Non-Hispanic) 25.6% Black or African American 23.7% Asian 6.0%People who assume Texas is guaranteed to be a red state forever are being naive. You push enough people over the border and it will be a blue state in 10-20 years. Dems know that, and that's why they are so nutso on immigration.