Certified GKH Gardening Expert
This could be a watering issue, either to much or to little. Check the soil. If the top inch or so is dry, they are ready for watering.
Container plants will need more water then if they are planted in the ground. In warm temperatures, you may need to water daily.
The plant also may just be aging naturally and beginning to die down for dormancy.
Yellow leaves also can indicate disease or insects. Inspect the plant for signs of these and treat with Neem Oil if you need to.