Did you slowly transition your indoor plant into being able to handle outdoor conditions? If not, it may be due to sun-scald or climate shock. While growing indoors your plant's leaves were used to growing at lower light levels than what it would have been outdoors. If only a small proportion of leaves are affected it should be okay, but if you notice more of them starting to turn, I'd say to find it a shadier spot for a week or so, or move it back indoors during the middle of the day. It won't take too terribly long for your plant to adapt and toughen-up, but right at the beginning it may need a little help.