It's an air bubble. After some time, cels start 
to warp and shift a bit, which is why it is 
always a good idea to separate cel layers if 
possible. Because of the warping, if you lay to 
cels on top of each other, they wont be 
completely flat and will have air bubbles in 
between them. Try laying a piece of glassine 
between the layers and it may reduce the effect. 
Whenever I am able to separate cels, I always use 
a thin sheet of glassine between the layers. Hope 
this advice helps, and if anyone disagrees wit 
this idea or has a better one, please say so.