Check apple trees for signs of canker. This fungal disease can work its way right around a small twig or branch, causing the top to die back. Fungal spores can enter a tree through pruning cuts - there will be fewer of these if you clear as much canker as you can a week or two before starting a full annual pruning regime
Cut well below the canker in case there is fungal spread inside the branch and check that the cut end is clean - if there are brown marks then cut further down again.