Brassia's will also periodically just jettison a leaf. If it's an older leaf I would not worry about it especially as you say the pseudobulb looks fine. It's the natural process of growing and aging.
I have a caudata in a 14" bulb pan that is currently holding 11 spikes of buds. I noticed a small brown leaf this morning and it fell free and I chucked it and won't lose a minute of sleep over it.