Thanks for writing! It depends on how many leaves are turning yellow. It may be nothing to worry about --- if it's just a few leaves every now and then, I'd guess old age is the problem. Once the leaves reach a certain point, they don't function well and the plant drops them.
If it's a fair number of leaves, then the problem could be too much or too little water, exposure to cold or warm drafts, too little light, or something like that.
I hope this helps!
Senior Garden Editor, BHG.com
Better Homes and Gardens