These are monsoon climate trees and must have moist roots in summer. Flagging and rolling of Kousa dogwood (and also Eastern dogwood) is very common in my dry summer climate. I've got a 'Big Apple' in the back yard that is a chronic problem child, currently repeated sprinklings have resulted in it making new growth at the same time that the mature foliage is droopy! This is not desirable as the added burden of the tender new foliage makes the plant even more vulnerable to being damaged (burned or even killed) by drought. When watering trees and shrubs during dry periods you want to put on enough to maintain them but not enough to promote flushes of tender new growth - unless you can then depend on keeping these adequately supplied with water from that point on. Where there is a problem giving the plant as much water as it seems to want at that time just to maintain its mature growth in good condition, it is better if there isn't new growth present as well.

Both are woodland trees (from humid climates) so it may also be possible that hot sun beating on the foliage or roots, or low humidity causes them to roll their leaves even when watering appears to be adequate. When I was a student at Washington State University, located in the semiarid climate of southeast WA I noticed that the only Eastern dogwood on the campus was in an enclosed area, planted in a an island surrounded by a pool.