To much sun ...too little water...to much water ...likely the either first two or both of the first two ...it also is not unusual for JM's to loose leaves or have tip burn especially in your area throughout the summer if it is hot enough even with well sited trees ...if it is just happening now at the end of the season consider yourself lucky !!!! It should have absolutely no effect on how it will look next spring unless it is diseased then it would likely have dieing branches... not dropping or tip burn leaves ..David