Yeah, I don't know what might cause that, but when I see any orchid giving up leaves, they better be making new ones or I get worried. The nutrients are stored in the roots and leaves, if it doesn't have enough reserves to make new leaves from the stores in the roots, they seem to give up old leaves to power the next new leaf.
It's not necessarily a bad thing if they're making new roots, new leaves and losing old leaves... especially if it's a very tall plant. If it has so many leaves, it has very little motivation to keep the bottom ones if they aren't producing as much energy as the new fresh ones.
I would say that it isn't a normal thing for the plants, but certainly common enough if there is a colloquial term for it. Because there are examples of very large plants which haven't given up their bottom leaves, example here of a vanda roxburghii tesselata:
http://www.orchidboard.com/community...tesselata.html
slightly smaller, but none-the-less all leaves seem to be intact:
http://www.orchidboard.com/community...ar-suavis.html
I think it becomes more difficult for the plant to absorb nutrients, and move them up through the stem of the plant as it becomes larger, simply because the stem hardens over time and the physical osmotic pressure to move nutrients and water upwards over greater lengths is harder... They can't grow for ever indefinetly either, I'm not sure what the end of life of a vanda would be...
I bought a fairly small Vanda denisoniana which has the palm look to it, and roots are very tough, almost impossible to get them to turn green. It only has about 6 leaves, but it has probably 30 or more spots where leaves used to be. I assume this was either wild caught at one point, or basically neglected (being a cool grower) and it shed leaves keeping only those which are the most efficient at the top. It's coming back, growing a new leaf very quickly, but I'm not getting any new root growth which bothers me.
So my experience says that it's basically not happy and trying to maximize it's energy efficiency to what is available. I'm not saying you're not doing a good job if all your others are happy, but the plant is the boss, not any of us. I'm not trying to put anyone on the defense, I just always believe that when I am having a problem with a plant, it's me, not the plant... not that I may be doing the best possible job and all my other plants love it, but there's something that I'm missing. Whether it's conditions or a virus/bugs... whatever the case may be, I have to believe that plants just don't become unhappy for no reason as part of their life cycle... but I've never owned a giant vanda either...
Maybe it's a cool growing or has other needs different than the typical heavily hybridized? What kind is it?