I see one in the photo. How many are there and did they all start to take on this appearance at the same time? Could be that their rootball is under attack or they have come to a natural end. You probably will never know the actual problem. Sound like they have given you years of enjoyment. My course of action would be to remove them and plant new ones though not in the same place just in case it is a root ball problem that is doing them in.

How long after they were planted did they seem to do well? Did the problem come on suddenly, or did it gradually develop? Had the grade been changed around them just before the problem developed? Were they badly root bound when they came out of the pots? What have you been feeding them, over the years? Lots of information needed to make an informed diagnosis, Cerifrancis.

Your soil looks a bit on the free-draining, stoney side. I would give them a feed with a little high nitrogen fertiliser; Sulphate of Ammonia or similar. A mulch with some old compost would help and make sure that they are kept moist during the summer. Give it some time before thinking about Box aphid; a brush of the hand will disturb them.

When you took them out of the pots they came in were the roots curled round and round in the pots, and if so did you try to uncurl them? If they had been in the pots too long the roots cannot always open themselves out (This is what Tugbrethil means by root bound)

Thank you, Steragram! :)
A little definition always helps!