Today was our warmest day in 2009 (upper 40s) so I went out to see if my suspicions were correct about at least one of the hives. I opened up Bee Glad..., separated the boxes, and, yes, BeeGlad... is dead. I didn't look all that closely or take out any frames but my initial diagnosis is starvation with plenty of honey above.
Both the top and middle boxes were heavy with honey (and possibly too much), so that the winter cluster in the bottom box had no empty cell area on which to move up to the top boxes. The dead cluster was still in the bottom box. With the freezing cold we've had, the cluster wouldn't have been able to move up much anyway. If, as beeworks.com mentions, the two top boxes had been clogged in fall, the queen might not have had enough room to lay eggs to replace the worn-out and dying summer bees. This would've left fewer bees to keep the cluster warm.
No matter which scenario, I must, in the future, pay more attention to leaving the middle frames in both top boxes relatively free or pollen and honey. When I had closed Bee Glad... in mid-October, I had left, what I thought, was a clear enough pathway up into the top boxes. However, two things happened: 1) I continued to feed the hive sugar syrup with Fumagilin-B which the hive consumed greedily, and 2) there was a stretch of warm weather at the end of the month in which the bees scavenged at the local Kwik Trip for sweets. The bees may have stored more honey right in the spaces I had left empty.
I am not very happy with myself right now.