Q.Why Are My Indian Hawthorns Leaves Turning Orange And Falling Off
The plants are well established but are getting then due to the leaves turning orange and falling off.

Certified GKH Gardening Expert
Check the pH and nutrient level of the soil. It could be a phosphorus deficiency, or soil that is too acidic.
Normally, the new growth will come out bright red, but it should not be sporadic like in that picture.
Here are some articles that will help:
https://www.gardeningknowhow.com/ornamental/shrubs/indian-hawthorn/growing-indian-hawthorn.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm