I'm now going to use that much overused word - stress, in this case environmental, most likely a result of the extremely hot weather we had some weeks ago. Excessive heat and hot sun can scald the leaves, but also its difficult to keep up with watering and these plants do not like to dry out completely. You don't say whether yours is in a pot or not, but if it is, if we ever get any really hot weather again, suggest you move it to a shadier spot and increase watering. The other common cause of orange leaves is too much water round the roots in winter, but obviously, that's not what's happened here. This does not mean you shouldn't water in winter though - the plant likes free draining soil, but not to the point where it dries out completely.