Digital BW, The Print

Just another guess -

The print touching the plex must have had an effect.  On the other hand,
cracking (of any material) is stress related and caused by either humidity
or atmospheric pressure (less likely).  

Atmospheric pressure (unlikely, but possible):  Where are you located?  What
elevation?   Where is the print located?  Elevation?  I might think about
the elevation of the paper factory; however, this problem has occurred
beginning from the elevation where the print was produced.  

Humidly :  I'm assuming that your client has an expensive house.  If he has
a modern (last decade) heating and cooling system, he likely has
sophisticated humidity controls allowing him to seriously de-humidify his
home.  If so, that could likely crack a Photo Rag.  Think about a print
produced on the coast in say Charleston, SC and then hung in death valley
(Racetrack?).  That print is going to crack.   

Another person's 2 cents -

Tom