Bougainvillea Winter Leaf Loss.

Bougainvillea they are in most cases semi evergreen plants and when the
light levels drop around the end of the year.The plant tends to loss
some of its leaves in the natural process of it yearly cycle.
Plants can also do this to recycle nutrients from old leaves into new
growth so its also worth making sure the plant is not pot bound and if
so the plant will need re-potting.
One way to check the plant is ok is at the joint of the stem were the
leaf has fallen check the axis bud to see if its still alive and moist.
One other problem might be that the plant is infected with a pest called
Red spider mite.
If no pests or root bound problems can be found as the day length picks
up the plant should start to produce new leaves.