Eric,
can you post a picture?
I have had a similar issue and it certainly seems to be some disease even though people on the forum thought it probably wasn't (see
http://www.orchidboard.com/community...root-loss.html for pictures). The bottom line for me is that this is spreading and when the issue hits the rhizome or the stem of monipodial orchids it travels up the stem or through the rhizome and eventually kills the plant. The plant looks fine at this stage, but eventually it starts loosing leaves and its vigour is affected. I dissected an affected plant - in this case a Renanthera (which I had two of). I found a) where the root issue hits the stem, the disease gets into the stem (grey or light brown cells in the cross -section), b) the issue moves slowly upwards.
I am not quite sure what it is in my case, but it could be Rhizoctonia (which is a little odd for plants that are not potted as Rhizoctonia is normally associated with decaying substrate) which clearly isn't an issue for mounted plants.
There are only a few diseases which can start infecting roots: Pythium and Phytophthora (Black rot) and Rhizoctonia.