This may be simple aging of your Orchid. Review the articles below to help you pinpoint the issue.
You'd really need to provide more information on your orchid in order to properly answer your question, but here is something from Orchids.com that could help you start figuring out what is happening: The answer to this question depends upon which leaves are involved. Yellowing of old leaves on backbulbs is a normal part of the aging process. Therefore, this should not be of concern.
However, yellowing of newer leaves is a sign of trouble. Orchids with yellow leaves are an indication that the plant is suffering from either too much light or insufficient feeding. Increase the shading and apply orchid nutrients as directed. Other causes of yellowing include loss of roots and stress due to low temperatures. http://www.orchids.com/Orchids-FAQ-W2.aspx#yellowingLeaves