How come my apples are falling off the tree before they are fully ripe?

It's what is known as the 'June drop' (though it's now July!) Most fruit trees like apples drop a percentage of the immature fruits now so that others can grow to a fair size. As long as the tree is healthy and there are fruit left, I would not worry. In fact, people are employed here at this time of year in the fruit orchards to deliberately thin the fruit so better quality ones are produced in the autumn.