Tire Blowout problems of the 2006 Ford Taurus

I had purchased a set of bfgoodrich premium touring tires from costco. I have had blowouts in 4 of 6 tires, all with around 45-55000 miles on them. The sidewall had cracks that went from the tread down across the sidewall. I have only had 1 blowout since I started driving since 1978. Besides these tires. I put 70000 miles on the original tires without problems. I do not understand how this many tires have blown. By the way, the 2 tires that did not blow out had cracks forming on the sidewall by the tread as well. (the reason I had 6 is because 2 were replaced under warranty) the "tires plus" store I went to did not understand why the tires were doing what they were doing, but he did say that it was not normal. I just want to help prevent accident or injury to others. It may just be the tire size that is affected. The tires were not old, the tires plus manager checked that first when he saw the tire. I have a set that were placed on my 1991 Ford tempo and they are not showing the same cracking. I only have about 30000 miles on them right now. 3 of the 4 blowouts were on the front and one on the passenger rear.