Most common cause is bacterial or viral leaf spot diseases. If leaves are spotted, especially if there's a white dot in center of spot, that's the cause. It's difficult to treat once started. Pick off & destroy the yellow leaves (burn or ziplock bag). A fungicide would help, but it will be uphill.

Do not wet the foliage of impatiens when watering. Water the soil & try not to splash soil on leaves when watering.

Al

Another cause is letting the plant get so dry it wilts before watering. Sure they do pick right back up....but very often some of the wilted leaves will turn yellow and drop.
Linda C