Strange Phal root die off

[snappyguy]

I have 2 miniature Phals, Sogo Gotris 'Flora Ark' and Flora Gigi. Both were bought from local Home Depots (2 different ones.) Both sit in the same west window, both are watered with 20-14-13 fertilizer weakly, and both were potted in 2.25" pots with fine fir bark and ~20% total of perlite and charcoal. Humidity is generally 40-50% this time of year. A few days ago Flora Gigi dropped a couple leaves suddenly. I checked the roots and most were dead, just an empty layer of velamin. When I checked Sogo Gotris the roots were like they were when I got it. I had checked the roots when I brought them home and repotted them from the sphag they were in into bark. Literally everything as far as culture is concerned is the same between the two plants. Any ideas as to what might have happened. Flora Gigi still has about 4" of roots left and the remaining 3 leaves look good. I think I will cut the spike shown in the picture so that it puts energy into growing again. Has anyone had something like this happen? Is it some sort of disease?


Sogo Gotris left, Flora Gigi right. Potting mix in center.

Mark