Digital BW, The Print

I just had a customer bring in a stretched canvas done with Cone's  
selenium inkset with portfolio black in December of last year and the  
top 1/3 has gone brown. I would like to blame it on sunlight hitting  
the top 1/3 of the image but even the black parts that are stretched  
around back have gone brown. Part of the problem is the image has  
been traveling to different galleries so no idea what the viewing  
conditions were. It came back packed in bubble wrap and a cardboard  
box (well packed) but certainly not archival packaging. I would like  
to blame the packaging, but why is only 1/3 of it effected?

Any ideas?
Mark