Sounds pathogenic. Examine the affected branches closely, for clues.

When you say pathogenic, do you mean as opposed to problems with plant culture (what I do to it)?
What clues would you look for? I'd look for signs of insects: webs,galls,eggs, munched. I'd look for disease, though I'd have no clue what I am looking at: leaf spots, burn, rot, dry?
Patterns: is there a pattern to the defoliation...all one place, all one side, what part of the season does it occur?
Are the defoliated branches dead?
and.....
I'm tempted just to hack it back and replant it in more sun. Then get another really healthy one and plant it in the same spot. If something similar happens to it, then it's location or culture