Yellow Leaves on my Orchid

I have a large Cymbidium orchid which I keep in doors all the time, out of the sun etc.
I have noticed recently that one or two of the leaves have suddenly gone yellow - does anyone know what might be the problem or is this normal.

 

If its the older leaves, on bulbs that have flowered Charlie, its not a problem. The old bulbs at the back of the plant eventually have no leaves at all.