Given that I've had this happen to me at a checkpoint (in Seattle) and had it explained to me, I think I have a reasonable potential answer: Stacked paper produ

Given that I've had this happen to me at a checkpoint (in Seattle) and had it explained to me, I think I have a reasonable potential answer: Stacked paper produces false positives in the scanning machines which can be flagged as suspicious substances. By separating the paper into a separate bin, it can be more quickly inspected by human eyes and the flag cleared, allowing the security line to be cleared more quickly.