...my guess is that it was placed in a humid 
place and never aired out...ewwwww. I don't know 
myself, but I did, however, find this nifty 
article on mold and mildew prevention in museum 
collections:

http://palimpsest.stanford.edu/bytopic/disasters/p
rimer/npsmold.html

 It says that cellulose-based objects are 
particularly susceptible; how much you wanna bet 
the cel is old enough that it's from when they 
were still using cellulose? ^^;;

-Misha

P.S. - What's that cel from?