This can be due to temperature changes or temperature ranges too cold or too warm.
Day time temps should be between 65 to 80 degrees F, Night temps 60 to 70 degrees F. Never let the temperatures fall below 50 degrees F.
Ethylene gas can cause bud drop. Make sure the orchid is at least 10 feet away from ripening fruit.
Watering issues can also cause buds to turn brown and drop. Too much or too little will cause bud blast.
Here is a link that will help refresh you on the care requirements.