Sycamore trees shedding new leaves in June

I was wondering why the Sycamore trees next to our garden have started shedding young green leaves already. They don't look diseased although they have green fly on them. They often fall after a storm, but we've had good weather. Is it because it is dryer than normal? We don't often have such good weather on the edge of Bodmin Moor, Cornwall! Amanda