Its the weather, Japon, the plants are nearly as confused as the gardeners. Actually, it is not unusual for some plants to have a second flowering period. The flowers are not as good or plentiful as in spring but they are trying to produce as much seed as possible to ensure survival. However, this year it is more noticeable.
It is interesting that you are asking this from Derbyshire. I was at a flower show in Inverness yesterday and this is one of the things that we discussed.

Hi Japon, i only said to my hubby the other day that one of our 6 rhododenrons is covered with buds. None of the others are!

A few of the evergreen types will naturally put on a second bloom in late summer, Japon. Rutherfordianas often do this where I am, though yours are probably a hardier series. And, as Bulbaholic says, the up and down weather the UK has been getting lately may have the plant's biological clock all confused.