Yellowing leaves generally indicate a watering issue. Either to much or to little.
Bottle Bush do need regular water so if it has been dry and with the summer heat it likely needs more water.
2 to 3 inches of hardwood mulch can help with moisture retention.
Here is a link to refresh you on the care requirements.
It does seem too early to change colors. Most buttonbushes turn yellow but there is a cultivar whose leaves change to coppery bronze in fall.
This could be a nutrient problem. I would have the soil tested at the local cooperative extension office.