Reasons for Phalaenopsis bud drop include:

a) Sudden temperature changes (drafts, etc.)
b) Underwatering
c) Low humidity
d) Damaged roots
e) Air pollution (tobacco smoke or other combustion
fumes, ethylene, etc.)

If low humidity is a problem, here are some tips from University of Minnesota Extension:

1. Install humidifiers in individual rooms or in heating systems. Keep plants away from radiators and drafty locations. Moving air picks up water vapor.

2. Place plants on a tray of wet pebbles. The water evaporates from the surface of the stones creating a more humid microclimate. The pots MUST sit above the water line. Placing a saucer beneath each plant will ensure this.

3. Group plants together so they benefit from the moisture that evaporates from them.

4. Place plants in areas that are naturally higher in humidity (i.e., near the kitchen sink, above an aquarium, or in the bathroom), provided they still receive enough light.

For more information, you may want to read: Care of Phalaenopsis Orchids.