Why would a healthy phal drop 4 of its 6 leaves?

[OrchidThief79]

Even though I've been growing orchids successfully for several years, I'm posting this in the Beginner's section because, well, I feel like a beginner and its a common problem: unexplained leaf loss of Phalaenopsis.

I bought a blooming, well-grown Phal recently. When it finished blooming, I didn't repot right away because I could see several thick, healthy green roots. Well, within a week, four of its six leaves began the "I'm gonna turn yellow and eventually fall off" thing. So, my first thought was "Uh oh, root problem." So I unpotted it, expecting to see rotten roots, but it has tons of thick, firm roots with no visible rotting on the roots or stem.

So why on earth would it drop so many leaves at once? I have several Phals. in the same growing conditions that are doing just fine. And if its some kind of disease, there are no visual signs of it. I just get frustrated when a well-cared-for plant suddenly decides to commit suicide.

Any ideas? Thanks.