The brown leaf looks like normal plant aging. When pseudobulbs reach a certain age, the leaf will turn brown and fall of.
Later on, the pseudobulb itself will shrivel and die.
In a healthy plant, you would have more new pseudobulbs growing compared to the ones that are dying due to old age.

Can you post a picture of the entire plant? Perhaps that would give us a better idea of what is happening to the overall plant.