It could be caused by the weight of the workpiece tipping it up into the abrasive when starting and exiting the sander, which happens to me on my lunchbox planer with longer boards.
If it's happening with smaller boards you can try gluing on some smaller pieces to the sides. This will put the snipe on them and save your expensive stock
If it's happening with smaller boards you can try gluing on some smaller pieces to the sides. This will put the snipe on them and save your expensive stock