Tire Bubble problems of the 1999 Subaru Forester

In one months time we have had 4 separate cooper weathermaster s/t2 205/70r15 tires have sidewall failure. These are snow tires used only during winter months. 3 of the 4 tires were purchased for the 2010/11 winter, and had less than 5000 miles on them, the 4th was new with about 100 miles on it when it failed. 1st blowout) drivers side rear tire blew out in driveway just after we returned home from errands. Replaced with new tire by dealer. 2nd failure) noticed bubble on drivers side front tire while shopping 45 minutes from home. Called roadside service to put on spare and when home, went to dealer who replaced with a new tire. 3rd failure) 2 hours from home, stopped for gas and noticed drivers side front tire with bubble in sidewall. This was the new replacement for the 2nd failure. Put on spare and drove home. That night we had our 4th failure - blowout) the passenger side rear tire blew out during the night. 3 of the tires were 2 seasons old, the 4th was a brand new replacement tire for one of the earlier failures. In all cases we were given brand new replacement tires of the same brand and model. We were fortunate and not driving when the blowouts happened. This all happened between mid November and December 16th 2012. All blown and defective tires have been returned to dealer and we do not have access to the dot number.