Tree loss/death can be caused by a number of things. The image you've sent shows some leaf margin necrosis - that could simply be buildup of salts from municipal watering during lack of rainfall periods. The 'dots' on the leaf are probably from apids of some type. Post oaks in our area also commonly die as a result of a fungal disease - hypoxylon canker. Death can be slow - tip branches dying back of a period of years or sudden - leaves turning brown quickly over a period of 2-3 weeks. There is no treatment for this disease, other than proper watering during periods of no rainfall.
Additionally many post oaks die due to root disturbance and soil compression - caused by trenching, new construction or even heavy vehicles parked on the soil where their roots are growing.
For a definitive diagnosis, we suggest that folks consult with a certified arborist

Why Hire an Arborist?

An arborist is a specialist in the care of trees.  The International Society of Arboriculture is a worldwide professional organization dedicated to fostering a greater appreciation for trees and to promoting research, technology, and the professional practice of arboriculture.  ISA Certified Arborists are experienced professionals who have passed an extensive examination covering all aspects of tree care. Arborists are knowledgeable about the needs of trees and are trained and equipped to provide proper care.     

International Society of Arboriculture – site listing for arborists:

http://www.isa-arbor.com/findArborist/findarborist.aspx

Certified arborists currently listed in Brazos County, Texas:

Jeff Lehde  - Plant People <personal data hidden>                                                             myplantpeople.com     <personal data hidden>                                                     

Dr. W. Todd Watson                                                             

PO Box 524                                                                                                         Millican,TX77866                                                                                                        <personal data hidden>                                                                                               <personal data hidden>

Andrew Hillis   Professional Tree Service  <personal data hidden>                                                                                                                                      <personal data hidden>                                                                                 For other arborists in the region, consult the local phone directory or the ISA website directory, where currently certified professionals are listed.

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