if it is newly planted then could also be lack of water and or root damage during the planting stage. Plants drop leaves as a response to 'shock'. its like our fainting :o)
keep it well watered and it may well recover. more likely to do so. so dont give up on it even until next spring.

Dropped leaves can also be a sign of waterlogging where the roots start to suffocate. If the ground is very wet (rain, rain, rain) try draining it. It may be too late if the plant has already drowned.