Question: We have native post oaks around our house. Suddenly one of them has died. Are there any diseases or insects we should be worried about?

Answer: Probably not. Post oaks are notorious for having “people problems.” 
They simply resent human invasion into their lives. If your house is fairly new, the damage done during construction is probably the cause. If the house has been there for 5 or more years, you might analyze any recent changes that might have occurred in the tree’s vicinity. Be especially careful when changing the grade, cutting roots, or doing extensive planting beneath the tree. If you can rule all of those things out, it may just have been its time. You probably should consult a local arborist to check the health of your remaining trees.