Premature colors can be an indication that a tree isnt strong enough to withstand insects and disease organisms that may attack it. Occasionally only one or two limbs of the tree will show premature fall color. This could be a sign of a disease at work, weakening only the infected limbs. The more common situation is for the entire tree to exhibit premature fall coloration, a phenomenon usually linked to root-related stress. Trees respond to these stresses by trying to curtail their above-ground growth.
If the leaves on your trees seem to have gotten a jump-start on fall compared with those on similar trees in the area, then you might want to consult a professional arborist, who can identify any problems and offer possible solutions.