Stacey started this thread on my behalf--I'm the owner of the chin who died (Chancellor/Chance).
I've had the same thoughts that many of you have had. Chance wasn't kicked or stepped on, and I can't imagine he was held too tightly (we hold them by supporting them from the bottom, allowing the chins to sit on our hands and against our chests). I keep coming back to the fall theory, but I just can't figure out how he could have fallen, injured his chest, and not suffered injuries to his limbs. (The vet found no bruising elsewhere during the necropsy.) His cage is large enough to house a few jumping ledges, but is far shorter than some of the "luxury" chin cages I've seen out there. I don't think he could have suffered a deadly fall in his cage. If it was a fall, it must have happened during his out-of-cage time. Since his death, I've only found one previously un-chin-proofed place high enough to potentially be dangerous (my youngest, most mischievous chin Lily was kind enough to show me), and I don't think Chance could have made it up to that spot (he wasn't as avid a climber as Lily). Still, this one high spot is the only place I can think of, so I hope that blocking it will be sufficient to keep my other chins safe.
If any of you have other theories, please keep posting. This is the second chin that I've lost and, although the circumstances were unrelated, I'm starting to become a bit paranoid about my chins' well-being.
Thank you for your kind words of sympathy and support. Chance was a wonderful companion and will be greatly missed.