I was out of town this last weekend (Memorial Day Weekend) and when I returned noticed all of a fairly abundant crop of small applets had fallen off of my Gravenstein apple tree on to the ground. My first thought was the possibility of a hail storm but I have several other apple trees and they show no sign of damage. This is a mature 15 year old tree that appears to be healtlhy.

Does anyone have any idea what might be causing the apple drop?

Sounds like "June Drop" -- in late May! Check this site:

http://www.ppdl.org/dd/id/june_fruit_drop.html

My Gravenstein's are set ultra heavy, but we're a little later than the rest of the Valley. I'm only hoping they drop! Otherwise I'll have hours of thinning later this month... I've three trees of Grav's, and ones a giant over 80 years old.

As long as there's something left, you should be OK

"June Drop" in May. Nature’s spontaneous abortion of fruit. Most were probably not pollinated sufficiently. Nature will also "drop" fruit if over set on tree. No problem.
BUT, as Viorn mentioned his tree has a heavy fruit set. Best to thin to one apple per cluster and every 6-8 inches so you will have a crop next year. Gravenstein is noted for biennial bearing, 10 bushel one year, 10 apples the next.

Ted