Why are my buds turning yellow and falling off?

This is referred to as bud blast, and can be caused by the following conditions:
The plant has been too dry between watering, causing it to withdraw moisture from the buds.
There may be some wide swings in temperature, where it may be too hot in direct sun, or the plant may be too close to an air conditioning or heating vent.
There may be some fumes in the air caused by paint, natural gas leaks, or other chemicals. Flowers naturally create their own methane and collapse after pollination to save energy for seed production. Certain forms of methane or ethylene may trigger bud or flower collapse.
Cattleyas in particular are sensitive while in bud to overwatering, causing the buds to actually turn black in the sheath.
All plants need an adequate amount of light in order to flower correctly. Placing a plant in the center of a room, on a coffee table for example, is fine for display during an evening of entertaining, but to maintain proper growth and flower development it is best to keep the plant in its growing area (near a window or under lights).

source:orchidweb.com