Probably a combination of lack of pollination and the unusual weather pattern this year. My early plum (Lizzie) was absolutely smothered in fruit last year but I can only see about a handful this year. The family pear has set very few fruit and two miniature apple trees (one a family tree with three different varieties) looks to be the same.
There haven't been many bees about due the the cold and wet, so I wish I had hand-pollinated them all now. Still, I expect a year off will do them all good (we gardeners have to try and look on the bright side, don't we?!)