Mike, You took the words right out of my mouth but since I'm here, I'll say it again
This is the normal life cycle of the plant and the oldest bulbs do eventually die off. I just acquired a catt that had been divided, not 1/4 the size of this one where the oldest bulb has turned yellow and as Jerry has pointed out, if and when it becomes soft, it'll be removed. Enjoy your plant, it's beautiful. Do you know what kind of catt this is?