When you say the whole end dies, do the leaves turn brown and crisp?
If so, then it's probably fireblight. When you trim the dead portion, make sure to cut well down below the browned part (the usual recommendation is 6") and make sure that the cut end doesn't show any brown discoloration. Be sure to sanitize the pruning equipment before and after each cut. (I just sprays/saturate with rubbing alcohol. I also spray the cut for good measure)
Don't leave any of the browned branches or leaves under the tree but bag and trash or burn them.
Are there any other apple or pear family trees in your neighborhood with the disease?