phalaenopsis rapid yellowing and total leaf drop

[micasi]

Lower leaves suddenly show indications of yellowing. Within a week all leaves have dropped. The upper leaves still appear healthy when they drop except at their very base. No crown rot. Roots appear healthy. Slight indication of rot right at base of leaves within the stalk. In the past 2 months this has happened to two otherwise healthy and happy established orchids subject to no stress, not moved, not overwatered, no sunburn, and orchids were not in proximity to each other except possibly at watering. I've continued to water the roots on the first orchid and they still appear healthy but dormant. Any ideas?