My Christmas cactus is losing large stems.
Certified GKH Gardening Expert
A Christmas Cactus will drop some of it's leaves if the growing conditions are less then ideal.
Lack of humidity and warmer indoor temperatures during the winter months can cause stress.
During the winter months they actually do prefer cooler temperatures.
Too much or too little water will cause leaf drop.
Water sparingly and never allow the roots to sit in water.
Old soil may not be providing nutrition and water uptake. Your plants should be repotted every 3 years.
Here is a link with more information.