Those yellow leaves may just be normal leaf drop and no cause for alarm. The reason I say this is because the other leaves look so green and healthy and because this time of year (late summer) it is normal for tropical plants like hibiscus to drop some older leaves. The most common cause of yellow leaf drop on a potted plant is too much water or the pot sitting in drainage water. Be sure pot drains well and water when top inch of soil is dry. Keep growing, Marianne Binetti