You may be seeing the effects of magnesium deficiency on your poinsettias. This causes the lower leaves to yellow around their edges followed by browning and curling. Eventually the leaves drop off. This begins on the lower leaves and moves up the stem. Eventually all of the leaves can be lost in this way. Usually applying magnesium sulfate at least twice a year (spring and summer) will prevent this deficiency.

Some photos of magnesium deficiency on poinsettia can be seen in this fact sheet: http://polkhort.ifas.ufl.edu/documents/publications/Poinsettia.pdf

Also, waterlogged soil conditions could also produced the symptoms that you have seen. Be sure that the soil does not remain constantly wet