From what I've found extreme temperatures can cause this...too hot or too cold.
Lack of pollination...fruits without seeds aren't producing proper hormone levels so the plant jettisons the fruit because it doesn't fulfill it's reproduction purpose.
My *very* rudimentary take on pollination is that each tiny grain of pollen causes a seed to form. The seed formation creates hormones which signal the plant that "all is well" and to "send nourishment, etc" to this fruit because it will secure the future generations of the plant. And, with the more hormones the healthier and larger the fruit grows.
I used to grow birdhouse gourds for purple martins and the trick was to heavily hand pollinate the female flowers...rather than 10-12 inch gourds you'd end up with 14"+ gourds due to the more seeds produced and thus more hormones.
This is all unscientific and simply anecdotal experience.
