Most likely drought

as Bamboo says, could be the prolonged dry season. How old & how big are your trees ? I planted 5 apple & 1 cherry in Spring 2009 & if I had not watered them frequently this year judging by the Grand Canyon cracks surrounding them I reckon I would have lost the lot despite them growing in the ground for 18 months.
I have heard that it can take 5 years or so for a mature tree to show the effects of drought.
Hope your trees will be ok.