Tire Bulge problems of the 1996 Nissan Pathfinder

Noticed there was a hug bump/bulge on a futura super sport GLS 255/70/15 tire on my SUV. The bump was about 7 inches long and about 1 inch higher than the normal tread. When driving at highway speeds, noticed car was bouncy/rocking. Took it in to pep boys who said it was a defect and replaced at prorated rate. I was lucky to have spotted. If the tire blew while driving, it would have been dangerous. Two days after having the defective tire (mentioned above) replaced, noticed a second futura super sport GLS (purchased at the same time) was showing the same defect. The bulge wasn't as bad but it was starting to form and grow. Took it to pep boys who replaced again at prorated rate. It seems like there is something wrong with these tires. What are the chances that of the 3 tires I bought in 2006, 2 have the same manufacturer's defect? I have photos of the defect if interested.

Noticed there was a huge bump/bulge on a futura super sport GLS 255/70/15 tire on my SUV. The bump was about 7 inches long and about 1 inch higher than the normal tread. When driving at highway speeds, noticed car was bouncy/rocking. Took it in to pep boys who said it was a defect and replaced at prorated rate. I was lucky to have spotted. If the tire blew while driving, it would have been dangerous. Two days after having the defective tire (mentioned above) replaced, noticed a second futura super sport GLS (purchased at the same time) was showing the same defect. The bulge wasn't as bad but it was starting to form and grow. Took it to pep boys who replaced again at prorated rate.