Hmmm. Yeah, very frustrating when they do that and there's no apparent cause. I had 2 phals that were doing something similar: one of them had roots that were going rotten, the other had nothing apparently wrong with it, but all of the leaves except for the two newest at the top eventually yellowed and dropped off. I sprayed and sprayed. It looks like it's stabilized now, but it hasn't done anything in terms of new growth yet. One of those orchid "wait and see" issues...