Your flowers are beautiful, you've got new growth, and the arial roots look healthy. From the photos I don't see any leaves dying either. That's all good. Now let's see what could be happening with the older bulbs. They do look remarkably dehydrated.
Sometimes old pseudobulbs will shrivel and loose roots as the new growth takes over.
The top of your potting medium looks fine. Have you checked whats going on in the root zone? If you haven't repotted in a while you might want to check the medium surrounding the roots. It may have decayed and caused root rot on the older bulbs.
If you've already repotted with new medium it could have initially been a little dry causing the older roots to dehydrate and expire.
These are just a few ideas that came to mind and it may be that none of them fit your scenerio.
Basically, your plants look healthy except for the older dehydrated bulbs. Blooms and new growth are signs of good health. Just make sure the medium hasn't decayed so it will support healthy roots on the new shoots.
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