I've heard many tire manufacturers state 7 years is the longest tires should be left on a car or trailer, regardless of treadware. Obviously there are many factors that affect tire life. It looks like the the belt broke. Many things can cause this like sun, heat, overspeed, freezing, underpressure, overpressure, sitting in the same spot (flat spotting), etc..
That said, I've also run BFG radial T/A's and never had an issue.