My dracaena has been beautiful until now. All of a sudden I find yellow leaves on a daily basis. It has not been moved and is still being the same way as always.
Since you say the plant hasn't been moved or changed, I agree with Anne, the most likely problem is over-wet soil. Marginatas have a habit of looking fine for months, while under the surface the roots are slowly failing. Suddenly rot starts to travel up the stems, and when it gets to the leaves, they start to yellow and fall off at an alarming rate. Are the stems firm and hard in appearance, or wrinkly and soft? If soft, I'm afraid the prognosis is not good, the rot in the stems is usually fatal.
Marginatas absolutely hate moisture around their roots. To be successful with them, you need to test the moisture in the soil, all the way to the bottom of the pot, where the roots are. Do not water until the soil has only the barest trace of moisture. Here are some videos that will give you a better idea about watering http://www.youtube.com/watch?v=YBBh0RPPqu0 , http://www.youtube.com/watch?v=Tf-8InSamYQ , and http://www.youtube.com/watch?v=9gxnQNJopqg
If your plant is not too far gone, it might respond to allowing the soil to aerate (if that is indeed the problem), and bounce back. It's happened before. If not, don't be afraid to try again. Marginatas are not the easiest of plants.
Is your plant going through a 'growth spurt'? Usually dracaena plants lose leaves as new ones grow. If you feel that the amount of leaves being lost is excessive, something has changed. It could be anything from temperature, moisture, intensity of light, root structure, any one of a number of variables. If this has happened suddenly, as you say, my first thought would be moisture. Wish I had a better answer.