My 20 year old Gold Medallion tree is losing excessive leaves this year. It is about 50% bare of leaves and the leaves have a dark brown spot on the very tip!
Established cassia trees are quite drought tolerant. However, drought is stressful on trees period and can cause leaf scorch. Trees with leaf scorch will exhibit brown tipped leaves and drop leaves. Here are some extension articles on leaf scorch with management strategies:
https://bit.ly/1ABDeP9 (PDF)
https://extension.colostate.edu/pubs/garden/02911.html.
For more information on cassia trees, please visit the following link:
https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm