This early in the season the browning is a result of low temperature injury we experienced earlier in the month.  Most of the state experienced temperatures dropping down below freezing around 10-13 of May.  Japanese Maples are prone to frost and freeze injury as new leaves come out in the spring.  New leaves will continue to come out on the ends and should not show any signs of this.  If conditions turn dry this spring into early summer make sure the tree is watered well once a week (if we have less than 1/2 of rain per week).  Tree should not be seriously harmed by this injury.  This is a fine leaved tree that does not do well with drought.  It wants moist soil and if growing in clay this becomes tricky since the tree suffers in poorly draining soils.  So if the tree needs water make sure to water without getting to the point where soils becomes saturated.