My bet is his--required by law--driving log is either incomplete or shows that he was driving illegally per federal and state regulations. Actually, I just found this site that says the driver had multiple violations in his log and had similar violations over the past week.
Court paperwork has revealed new details about the multi-vehicle crash in North Austin on Thursday night that killed five people, including a child, an infant and an elderly woman.
www.fox7austin.com
Also, the charges against him--currently--all include that he was DUI / DWI at the time.