Yellowing leaves usually indicates overwatering. Dracaenas don't need watering as much as some other houseplants. In the summer, I water (and feed with Baby Bio) my dracaenas every 2-3 weeks and in the winter I water (not feed) every 3-4 weeks. I've had dracaenas in the past that I have overwatered and subsequently all the leaves fell. Also, dracaenas need quite a lot of light (they do best by a sunny window but not in direct sunlight) - have you recently moved the plant to a low light position? Yours is a lovely specimen - I'm sure if you ease up on the watering now you'll prevent any further leaf fall. If they continue to turn yellow though, have a look at the roots (if there are any poking through the bottom of the pot) and see if these are brown and a bit mushy? If so, you might need to cut off these dead part of the roots and re-pot the plant.