Yellowing leaves on angel's trumpet can be caused by quite a few things. Either over- or underwatering can cause yellowing leaves. If you've not changed your watering regimen, that may not be the case. Insect feeding could also cause yellowing. Examine the plant closely for aphids, whiteflies, or scale insects. Nutrient deficiency (especially nitrogen) can cause yellowing of leaves, too. However, nutrient deficiency usually comes on gradually, rather than suddenly. An abrupt change in environmental conditions can also cause leaf yellowing.