As follow up to responses to my original posting, a few extra pieces of info;
1. The Nokian Hakka C's are van tires with rated load factor 102, that's a weight capacity of 1874 lbs, so should be fine for a 2 tonne loaded Westy.
2. All four tires had the problem, worst was almost 300 degrees of the sidewall, least bad only 90 degrees.
3. The problem appeared within a single month between a wheel rotation in late August (hard to believe I could not have noticed such major cracks then) & after another 1500 miles of mostly highway driving to early October.
4. They were always inflated to 44 psi as recommended by the installing shop. 
This shop now recommends increasing pressure to 48psi on the replacements (another set of the same tire). I'll be watching them more carefully. Max rated pressure is 65 psi.
5. This local highly rated Vanagon specialist parts supplier & service shop gave A++ service in dealing with the problem & in working with Nokian to get replacements & some (pro-rated) $$ back. I've always received exemplary customer service here.

Michael Hart
Palo Alto, CA
'87 2WD Westy ('T3WESTY')
mjhart853@gmail.com