It could very well be due to high temperatures. Especially in container. Make sure to give these plenty of water and shade on hotter days. Sometimes pruning just the tip off of each branch will help force leafing as well. Just make sure to keep in the shade after you do this. At least until you notice a recovery.
This article will help:https://www.gardeningknowhow.com/ornamental/shrubs/burning-bush/burning-bush-care.htm