I would speculate that temperature is, indeed, the culprit.
Phalaenopsis, for the most part, are "hot growers," rarely seeing nighttime temperatures below 22-25oC. In winter, when humidity is low, and thermostats are pushed down to save on utility bills, evaporative cooling from the open medium can drop the root zone temperature significantly. Wet and cold is a sure way to kill phalaenopsis roots.
Consider putting them oin a seedling heat mat.