How strange, Yellowing of leaves on an orchid is a natural procedure but usually only one at a time when they are then removed. Obviously you have flushed it with feed and water when its been drying out, as you seem to know how to care for them. You don't think you have over-watered, as I read that watering too much is the biggest killer of orchids. I let mine get quite dry before feeding and watering. I do hope you don't lose it.

Thank you - perhaps I did over-water when I first saw the buds starting to open. I don't really know how to care for them to a fine degree as they have been successful for me by just using instinct. I just got a fright when I saw the yellow leaves ( which DID come one at a time) Many thanks for your help :)