Hmmm, the best way I can describe it, is to give an example of why humans get colds during the winter. Cold weather is not the cause of a cold, but the incidence arises in the winter for a couple of reasons...1) There is usually a sudden drop in temp, more than about 10-15 degrees and the immune system is getting stressed...2) We spend alot of time indoors, with others and more germs are passed back and forth.
Disclaimer: I am not a doctor and that analogy could be totally wrong.
So the acclimation isn't exactly what kills it, it's immune system is compromised by the acclimation. It is in a closed system whih is chock full of bacterias and viruses, etc...that it would normally fight off...but because it is weak it succumbs. It more than likely died of a bacterial infection, brought on by an accelerated acclimation.