Ask Mr. Smarty Plants

I suspect that you are seeing cultivars of pecans which may be much less resistant to stress and diseases, although “native” trees are possibly also being stressed to the point of losing their leaves.  “Native” pecans are recommended for yard trees because of their greater natural resistance to stress and disease and because they don’t kill themselves by producing more nuts than they can actually support.  “Native” in this context refers to trees that occur naturally and are not cultivated varieties. 

Second, I’ve seen lots of pecan trees that lose leaves and also limbs when they don’t get enough water. The leaf fall is most likely a result of the current drought following on the heels of ample spring rains.  I’m seeing lots of area trees shedding leaves as the drought goes on here.  The excellent winter and spring rains we had encouraged lots of lush foliage on our trees.  As a natural defense in the face of diminishing water availability, local trees are doing what plants do in this situation, they’re defoliating to reduce transpiration and protect the organism. 

Third, through researching the problem of early leaf abscission – what you’ll find in scientific writings describing leaf drop – there are several other possible causes:

1. Nitrogen deficiency causes leaves to yellow and in serious cases to fall off. 
2. Downy spot can cause early leaf loss in susceptible cultivar like 'Stuart', 'Pawnee' and 'Moneymaker'.  It’s possible our wet spring gave this fungus a foothold.  
3. The Twig Girdler insect can cause small twigs to drop in late summer. Check to see if  the leaves are attached to a small portion of the twig. There will be a perfect circle cut in the bark if this is the culprit.  

This question falls under the bailiwick of State Agriculture Extension Services and I found several useful  on-line publications.  Check out the following links: 

1. From Texas A & M – Evaluating Pecan Problems2. 
2. From the University of Florida - The Pecan Tree  
3.  From the Dirt Doctor  who researches organic growing methods here in Texas – Pecan Tree

So, although I can’t give you a definitive answer, hopefully this information will assist you in determining why pecan trees in Austin are shedding their leaves prematurely.