It seems you have more than one new growth. I think it's normal for the oldest bulbs to die as their reserves are being consumed by the new growths. After all that's where the plant stores energy.
But let it dry out can make it happen faster, I think.
I'm no expet on Zigos so let's see what others have to say.
It looks like normal die-back to me, possibly accelerated by the dehydration. If it is soft remove the whole pseudobulb to keep any rot from spreading, but if it is firm/dry it can stay until it dies back completely. You've got healthy new growth and roots coming so the plant should recovery quickly from any lingering effects of dehydration, but the old pseudobulbs may never plump up again.