Ask Mr. Smarty Plants

Any time the condition changes of a plant that has done well on a site for years, we have to ask what has been the change in the environment? Ilex vomitoria (yaupon) is a hardy native shrub. So, what has changed in your yard or what has been done that is different than previous years? The first thing that comes to mind is indiscriminate spraying of herbicides. Someone, you, your neighbor, a landscape contractor, may have been spraying weeds and a breeze drifted some of it onto your yaupons. Weed killers for lawns kill broad-leaf plants, or dicots, because grass is a monocot. Yaupon is a broadleaf plant. The weed killer has no internal instructions about "this broad leaf, not that broad leaf." 

Although yaupons are resistant to almost any natural problem, the bark can be damaged by mechanical impact, such as from a lawnmowever or edge trimmer. If the damage was not too extensive, the plant can probably recover, but you should take steps to avoid that happening again. You did not mention detecting any insect activity, but some of the possibilities are scale (eHow), leaf miners (PlanetNatural), mites (West Virginia University Extension) and aphids (University of Kentucky Entomology).

Since we can't possibly diagnose or recommend treatment at a distance, we suggest you get in contact with the Texas A&M AgriLIFE Extension Office for Harris County.  They are far better equipped to deal with pest and disease problems in a particular area than we are.