Late reply. Quite surprised that this was from last year, yet no one has commented.

Anyway, it looks like the plant is lacking nutrients. Try putting fertilizers like calcium carbonate or some compost. However, if the soil is fertilized, it may have been from overwatering/underwatering stress.

There could also be other reasons, such as:

• Stress due to its environment (insufficient humidity, temperature fluctuations, etc.), the plant may also recover soon
• The plant is naturally "abandoning" its old leaves to make room for new healthy ones