If they have suddenly gone from a shadier environment to a sunnier one, could be the sunshine (you are in a hot, sunny place). It could also be a loss of humidity in the air, but doubt your humidity is ever that high in the summer. They looked more dessicated than sun-burned. If they were very new leaves, than they got fried. New leaves are more delicate than older leaves and respond quicker to heat, sun, lack of humidity.