Any tire can fail from a defect in manufacturing. The tread separating from that Kumho doesn't mean that the other four on his trailer are bad, but I would be tempted to replace them as well. Used trailer tires are easy to sell.
Also, I wouldn't condemn a tire manufacturer from one bad experience. Kumho and Maxxis make fine trailer tires. Perhaps over-priced, but a fine tire none-the-less.