Not only are those leaves having to adjust to different sunlight, air currents and humidity but also to temperature differences. Keep them out only for an hour at a time and in shade to begin with. This allows them to harden to the air currents ( wind) and the humidity changes first, then you can gradually introduce them to direct sun for lengthening times each day after that. It is not just the sun you have to be careful of when introducing in home grown plants to the great outdoors.

Last year, my indoor started datura did the same thing. I guess it got too much sun too soon. So I pulled all the leaves off. It regrew new ones that didn't droop. It grew into a huge plant by the end of summer.