I would say that it's most unusual. Piper is a maincrop and I wouldn't expect them to start dying down til mid/late August at the earliest.
Have the plants been diseased or stressed in any way - blight or blackleg ? Drought or flooded ? Are the leaves and stems still healthy looking ?
I would lift one plant and see what you find. If you can give us some more info,or post a piccie of the plants, we may be able to help further. It may be that there is nothing wrong with the plants - when did you plant them? It maybe that if you got them in early, they have already matured - have they flowered yet ?

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Rat

British by birth
Scottish by the Grace of God

Blog updated Wednesday November 13th