The change appears to be caused by both decreasing temperature and decreasing light duration during the autumn. The change seems to be triggered more generally by the tree preparing to drop its leaves, and attempting to reclaim as many nutrients as possible from them before it does so. Thus other sources of stress, such as drought, can trigger color change as well.

There are two types of pigment visible in autumn leaves: yellowish carotenoids--which are present in the leaf throughout its life, concealed by the green of chlorophyll--and reddish anthocyanins, which are actually produced in the autumn. Why trees should expend the chemicals to produce anthocyanins is still somewhat mysterious, but there are two prevalent theories. The photoprotection theory holds that while chlorophyll protects the leaf from damage caused by sunlight throughout the growing period, the anthocyanins help protect the leaf while nutrients are reclaimed after the chlorophyll is used up. The coevolution theory holds that the color change is a signalling mechanism to warn insects to avoid the tree when it is stressed.

(Wikipedia)