Sometimes, flowering cherries will drop some leaves in the summer. This often happens if the summer is very hot and/or dry following a cool, wet spring (like we had this year). Cherries can also drop leaves if the roots are staying too wet, which can happen if they are planted in lawn areas and they are receiving heavy irrigation meant for the turf. You can tell if one of these situations is the reason for the leaf drop if the leaves are yellow, but not covered in brown spots. As long as the tree is not dropping more than 50% of its leaves 2 years in a year, it should recover just fine. If the tree is completely defoliating, has dead twigs or branches, or is covered in leaf spots, these may be symptoms of a more serious condition.
Cherries should have dead branches pruned out during spring or summer once the tree is fully leafed out. They do not need any kind of trimming, unless they are near a structure or signage. They are often short-lived trees due to the amount of diseases and pests that can affect them. Watch for boring insects, which attack the trunks of trees under stress.