Thank you for your question.  Other than the normal leaf drop following fruit production, the three main causes of apple tree leaf yellowing are (1) too little water, (2) too much water, and (3) deficiency of one or more nutrients.  I am unable to see an individual leaf clearly enough to detect a pattern which would show which nutrient(s).  You didn't indicate what your watering and/or fertilization habits are, so those sources of information are unknown.  I can find no disease that causes this type of "flagging" (death of only one or two branches) in apples.  You don't indicate the variety of apple you have, and some varieties are more susceptible to diseases than others.  And I am uncertain of the age of the tree.

So, here are your options.  You can have a soil test performed to get information and advice on pH and nutrients (https://catalog.extension.oregonstate.edu/em8677).  You can submit leaf samples and photos to the OSU Plant Clinic (https://bpp.oregonstate.edu/plant-clinic) to get their feedback.  Or, you can do nothing except keep your tree adequately watered, and heed the fertilizer advice found here:  https://extension.oregonstate.edu/gardening/techniques/fertilizing-fruit-trees

I hope this is helpful.  Good luck!