Hypericums are remarkably pest and disease free. The only possibility I know of in that respect is 'rust' which would appear as orange or brow spore masses on the stems and leaves.
My thoughts are with a period of dryness at the roots which if prolonged would eventually result in dropping leaves - likely to first turn yellow.
If you are on quick draining light or sandy soil I would make sure the surrounding ground is thoroughly soaked - which I'm sure you have already done, and then apply a thick layer of garden compost or other mulching material over the immediate and surrounding area.
Conversely a waterlogged soil will also produce yellowing of the leaves and subsequent leaf fall.