As the plant flowers, older leaves are drained for nutrients that are moved to the top and then they die and fall off. Pulling them off disturbs the plants natural cycle.
Your plant looks great, but if the yellowing and dying of leaves becomes extreme then you simply add a little N (nitrogen) fertilizer to your water. If you need to save money you can simply pee on the ground around it, or in your water, since urin is high in N. Just don't overdo it.
Remember that when the plant is ready to cut pretty much all of the large fan leaves will have fallen off and that's normal.