If its just a few at the bottom, that's probably just normal leaf loss, if its more than that, only thing I can think of is shortage of water - they like well drained soil, but usually choose to grow near riverbeds, where there's a good supply of water.

You could try watering with epsom salts, I did this with my Yukka a few years ago and it really perked it up ! my friend bought one with me and mine got much larger than hers, I only used it now and again.