Not a clue if this is even possible, but to me 
it looks like line fading.  Do you keep the cel 
on display?  It could be light damage.  But if 
it is possible for backgrounds to "eat" through 
the paint on a cel... that's bad.  lol  I 
normally speparate the cel and background so 
there is no way for the two to stick, but if 
yours was already stuck when you got it...  =\ 
*shrugs*