Blistering Tyre

The Metzeler Tourance Next front tyre on Stallion started showing weird looking bulges at places around 6 inches apart. It seemed like the tyre was getting ready to burst, although there was some tread still left on the tyre. When we removed the tyre the bulges persisted. Upon closer inspection we found that the outer surface of the rubber had somehow blistered and the inner surface was strong as ever.

I've used my fair share of motorcycle tyres but have never come across such a thing. Thoughts?