Thanks for your responses, all of you. My responses to you...
I was actually in the process of sealing the edge. What you say is correct, but we use industrial grade glue at 200C to glue tape on to the side of the board. It is a well proven process and provided it is done correctly does stand the test of time and protects the board.
nice idea, but that costs money and you know what management do when that subject comes up.
The panel standing on something wet was one of our thoughts, but we can find any candidates for that "something wet". There were about 5 panels out of 50 that were effected, and they were spread through the job. Panels next to the one in the photo had more normal swelling in the 0.2-0.3mm range so the trolley they were on appears unlikely to be an issue. I then wondered if they were all cut from the same sheet of whiteboard, which was already effected before hitting the CNC. I know they went straight from the CNC to the trolley - I watched them - so the damage would then have to have been before hitting the CNC. Possibly, but we never use the very edge of the sheet as it is nearly always damaged in some way from transport. So if the full sheet from which they were cut was effected by moisture, then it was likely that the effected edges would have been cut away just as part of the standard cutting procedure. Having said that, it is one of two theories that I favour. The other is a fault in the manufacture of the full sheet of whiteboard which made it more susceptible to humidity.