Wikipedia offers by way of explanation that the chain had been named Z-Teca up until 1998, when trademark infringement lawsuits by Z'Tejas Southwestern Grill and Azteca prompted them to pick some safer name. Z-Teca, meanwhile, was the name they picked after their original name of Zuma was also hit with a trademark infringement suit. How Qdoba made the cut I don't know but I'm guessing they picked anything that nobody had registered first.